In my experience, if you flash a new ROM before a phone has fully registered and pulled all the settings for data from Sprint, flashing a new ROM will cause this kind of problem.
I had it happen when I got a replacement device and immediately flashed a new ROM. If all else fails, this should fix it:
1. Flash stock ROM
2. Hard Reset
3. Go through all the customizations and make sure the phone activates
4. Verify data and phone working (If not, use the update profile and update PRL)
5. Flash custom ROM of your choosing
6. Hard Reset
7. Enjoy!
