Make sure you don't have too many splitters on your cable line. I had that problem. It would work fine, but the upload signal was way off. Log into your modem's IP from your computer through a web browser and get your upload dB and other stats. It will give you an idea of whether or not you're within the threshholds...
Ohh yeah, btw....I've had this happen a couple of times at my place. It's real picky and I blamed Comcast constantly.....until I reconfigured my splitters and it fixed the problem. If you have any splitters at all...take the cable when it first comes in and put a 2 way splitter on it...run one split to the modem and the other one to your other splitters. This will give the modem as much signal as you can spare right off the bat...