When you are reporting an intermittent problem, it makes it really hard to troubleshoot based on what we have before us.

Is it just the one airport?  If I were having this problem, I would be trying many different airports to see if there was something odd about the one.  It can be as simple as reading the wrong chart, which would result in the wrong ILS frequence if you were doing it manually.

If I tried several different airports and it worked fine, then I would conclude there was a problem with the one airport and either avoid it or try to figure out what is going on.

If you are having the same problem no matter where you fly, then roll back to when it was you were having great success.  Is it possible you are doing something different?  Have you made any changes to your system since then?

If you are a very experienced simmer with these kinds of aircraft, then you can fake it pretty good, but you are going to have to use the manual to get serious.  I printed out 2,3 and 5 and studied the others on screen.  2 is your expanded checklist that has the steps you need to be following.  3 is technical stuff that you probably need to know.  5 is your FMS.  It is 99 pages long, but mostly in outline form with illustrations.  It isn't really a huge task to print out and is relatively easy on ink cartridges.

Screen shots would help, if you can do that.

Are you obtaining your charts and following them?  How far out are you intercepting the extended centerline of the runway?  What speed and flap setting? Do you have LOC button pushed, correct frequency and course set for ILS?  Does LOC appear on your PDF?  Does it turn green?  We have to figure out where in the whole sequence things are getting messed up for you.