There is a limit on how much xp resupplying friendlies will actually grant. After some predetermined amount it will quit rewarding you though people can still resupply. It then goes on cooldown (not sure for how long though it generally seems pretty quick). If you're permanently not getting rewarded then it might be a bug that I haven't heard about in which case try logging out and then logging back in. That usually fixes these sorts of issues.
This ^^ You get appx. 20-25 refills, and the xp total may vary depending on the current bonus percentage. The time window is appx. 5 minutes. However, just do what I do all the time. As soon as it caps out, just die or suicide, and it resets it. Typically I just step out into enemy fire, usually near medics if I can, or I jump off an airpad or something. You can keep it rolling in nice and fast if you don't care about k/d, which I do not.
