FMC Flight Plan Manual Changes 757-200 v4.3

Once I've completed a flight plan in the FMC/CDU and review the leg speeds/altitudes I've altered some of the speeds and altitudes manually in the CDU. Following those changes the EXEC button was pushed. During flight the manual changes were ignored and the intial computed values were followed. Does the CaptainSim FMC/CDU accept and use manual changes?

If so, I'd appreciate tips on what should be done.