Songs missing from playlists

Hi ~
Apologies if someone's already asked this, but I've been trawling and can't see the topic.

Situation: I have an iPhone 4 running 4.2.1 - with no problems at all. I sync it every evening as this is my normal way to charge it up. I'm running iTunes 10.1 (54) on my iMac running 10.5.8.

Problem: My iTunes playlist 'favourites' holds 79 songs. In my left hand pane, when I open the drop-down list and look at the 'favourites' playlist as it exists on my iPhone, it only has 77. The same thing has happened to literally a few songs from my 'library' playlist, where I keep everything and the kitchen sink.

Question: Why is it ignoring random tracks and only keeping 77 out of 79? The other two tracks USED TO BE on there. I suddenly realised I hadn't heard them in a while, went looking for them, and they're not on the phone (under 'search' or just the songs list). On iTunes, they're present, ticked (checked?), have artwork and even lyrics. They are not present on my iPhone, even though they should appear in two separate playlists.

I've tried ticking and unticking the 'sync only ticked songs and videos' a few times, tried deleting them from iTunes and then syncing the phone before adding them again - but they don't even appear in the 'recently added' playlist, as all my purchased and CD-ripped stuff does.

Does anyone have any idea at all why this is happening? It was like this before the 4.2.1 upgrade.

Thanks in advance, anyone.

🙂