I had this same problem and here is what I found sorted the problem:
Open itune and go to "music" in your "library",
Make sure you are in "Songs" view by selecting the tab and make sure you sort it by artist,
Right click the bar at the top of the song list and make sure you have "track number" selected,
Find the offending artists and have a look at the track number and make sure they are the same format (some might be "1 of 9" format and some might be "1"),
To change all the songs track numbers in your Library to the same format:
Select all the songs and right click to "get info",
Select the box to the right under "track number" and click ok.
It will now start sorting them, this could take a few minutes.
Now move the music you want onto your iPhone.
This done the trick for me, hope it helps you.
If you don't understand something here let me know and I'll do my best to help you further.