I'm going to guess that the problem you are having is that lyrics don't always sync to the song tracks on the iPhone. I don't know why this happens or the easiest way to fix it. You can verify that this is happening by looking to see if the lyrics actually exist on the iTunes file. If they do, then the problem is with the syncing.
Suggestions I have seen and I'm wondering if anyone has better ones are:
- delete the song on your iphone and put it back on
- restore your iphone and have it sync back the library (seems pretty serious)
- identify the specific songs in questions and make minor changes to the file (ie change the name of the file, add something to the lyrics, etc). The idea is to force iTunes to resync and replace the file with the correct one.
Hope that helps.