The steps that I have sent before and at this post should get your engines started.  I have revised them somewhat to try to make them more clearer.
These steps should work unless you are trying to accomplish some other task with the engine start procedure.

Simple 17 steps for the Captain Sim 757 start up procedure.  Cold and Dark start.

1. Make sure all buttons are off.  Turn off ALL gauges, instruments and switches followed lastly by the Standby toggle and battery switch.  In other words nothing should be running and every guage, press buttons and toggle switches should be in the OFF postion.


2.  Begin the start up by turning the battery switch on.
(After the battery is turned on, within about 20 to 30 minutes, a significant amount a voltage will be lossed, so try to get the engines started within a 20minute time frame)

3.  Turn on STBY Power to Auto.

4.  Turn on APU Gen, Bus ties, L and R utility bus followed by the L and R Gen Cont.

5.  Turn APU (under gen drive disc) to start.  This APU toggle button will automatically switch to on.

6.  Wait for a steady RUN LIGHT that is at the left of the APU toggle switch.  Note that the light may dis-appear a few times before it becomes steady. When light is steady, proceed to step 7.

7.  After steady light, turn on the APU/Isolation (air bleed)valves that are located on the far right side under the duct press psi gauge.  Press the bottom 3 switches to on then press the single switch above the 3 bottom switches to on.  Note that the off light may remain lit along with the on light.

8.  Turn the eng start switch to 2 that is in the middle of the over head panel. The 2 is for the Right engine and the number 1 is for the left enigne.

9.  Proceed down to the eng start toggle switch and turn the RIGHT engine toggle to GND.

10.  Proceed down and turn on the left and right pumps (both AFT and FWD).  Leave the fuel cross feed to off.
NOTE that you can leave the left pumps in the off postion until you get to the Left enging start up process

11.  Press the “P” Icon to display the thrusters and bottom L and R  fuel knobs.  Click the right or left fuel knobs to open fuel line.  Upon this the engine should crank and start.

12.  Repeat steps 8, 9 and 11 for the left engine start.

13.  After engines have started,  turn OFF ALL Switches that were turned on in step 7 ONLY.  NOTE that other switches and guages that were turned on in steps 1, 2, 3, 4, and 11 should remain ON

14.  Next, turn off the APU Toggle switch. (step5 ONLY)

15.  Next,  turn on all HYD Switches , L and R packs.

16.  Next,  turn the rest of the switches and toggles switches that are NOT stated in the above steps , in according to your circumstances.  

17.  That’s it. Have fun.

I think you've encountered a bug that if you let the plane sit too long without starting the engines, they won't start no matter what you do.

Are your packs off?

Ah.... That's  a pain, but it would explain it.

A few notes about air...

First, if your APU "RUN" light isn't solid, even if the generator is going, you're not going to get the start valves to open.  I've encountered this rarely, I've had APU running, using its generator for power, got air at the panel, but no RUN light, and so the start valves never open.  Reloaded the sim to fix that problem the few times I've come across it.

Be sure to delete your "Default" flight (whichever saved flight has the "Default" checkbox set) even if it's for another plane, if that problem persists.  The default flight saving seems to save some CS settings even if it's saved without a CS plane.

Next, there's several things to keep in mind... If you're taking off bleed air anywhere, the start valves won't open.

Both pack selectors should be OFF.  Not Auto or any of the N/C/W settings.

All three de-ice buttons should be OFF.

All left/right fuel pumps should be ON. (center pumps will remain off regardless of command until engines come on)

Usually, if you are on APU power, have a RUN light, and turn the start valve switch for an engine, (and assuming you have APU bleed ON and Isolation ON if the right engine is what's starting) then double-checking these items will cause the start valve to open.

Obviously the plane needs as much air as possible to acheive 18+% N2.  It can't get that if you have de-ice or packs on, and obviously it will never fire over if the pumps are off, so it won't even open the start valve until it knows your fuel selector can actually provide fuel to the engine.

Thralni,

Check my tutorial videos, the topic about that is near the top of the list on this forum in the top section.

You have probably one of two issues-

1) Is the APU RUN light on solid?  If not, you have an issue with the APU.  Check to make sure there is no APU fire warning, and also try deleting all the saved flights that have the 757 on them *and* the default flight (even if it doesn't have the 757 saved) and then restart the SIM.  Sometimes there is an issue where something "weird" gets saved, and even though the APU is giving power and air, the RUN light never goes on solid, and if that condition is not met, the start valves won't open, or
2) There may still be something you're missing.

My videos cover all pre-flight steps, including setting up the APU and all air-driven items, and the startup steps, including cutting off air-driven items that are enabled as part of the pre-flight.

Some things are run by air, and people don't realize, such as the anti-ice systems.  If any are on, or if they're commanded off and you still have VALVE illuminated, then your start valves won't open ever.

Hope it helps.