If you are a member of the Steam beta program, it might be worthwhile opting out of the beta program and reverting to a previous version of Steam (it should do this automatically) to see if this resolves your problem.
Another thing that is mentioned in the Steam forums, is allowing Steam to verify all of its files and then closing Steam properly (rather than letting Windows kill it as it shuts down).
If you want to make sure you give each track at least a couple of spins before it disappears off your phone, adjust the rule to "less than" and enter 2.
But I recommend starting at 0 – it ensures a higher turnover of new music every time you sync.
When you click "Apply," these new settings will wipe out all the songs (if any) you currently have on your phone and replace your i Phone's entire MP3 library with the tracks in these new playlists.
So only click "Apply" if you're ready to change all the music on your phone.
Next, click "Live updating," which lets i Tunes adjust this playlist as you add music and as your play counts change.
Finally, name this list "Recently Never Played, for i Phone."Step two ——–That Smart Playlist loads up the i Phone all the brand new music you've never heard. We're going to make a separate playlist for the solid gold hits in your collection, a regular ol' dumb one called "Bona Fides." This is where you put all your Zeppelin, Pink Floyd, Stevie Wonder, and the Pixies – basically the music you can't live without.
If you're still having problems after trying all of the above, there are really only two options; @Andalur that isn't what your question says...
If the window appears every single time you start up Steam, you're probably experiencing an issue with Steam realising it has updated.
It's worth noting that this is a well discussed issue with no official solutions available, there are plenty of different suggestions on how this can be fixed though and some of these work for some of the people experiencing the problem some of the time, so you'll want to try all of them if you really want to stop Steam from verifying every time you start it.
Either way, if your i Tunes library regularly gets bombarded with new music, it's easy for all those incoming files to get lost in the shuffle – particularly if you do the majority of your music listening on an i Phone. Here's a super easy way to make sure all the music you've recently acquired (however you've acquired it), shows up on your i Phone every time you sync.
Set it up once, and never neglect a new band again.