I think Matt's right. I process a lot of paper negatives and I've been reading the experience of other people processing x-ray film over at F295. It sure sounds similar. With paper negatives the trick is to pre-soak them for a good amount of time, at least 2 minutes, and you must make sure the paper hits the water in a smooth motion, without splashes.
Even after the pre-soak, the first few seconds in the developer is critical. It needs to cover the negative quickly and smoothly and the pre-soak gives you a little more time to do this ( without the soak, it is very difficult to avoid uneven development marks. )
Uneven development can also be trickier to avoid if your developer is dilute or cold. I do that on purpose sometimes, but then I need to be extra careful. I occasionally develop in dektol diluted 1:8 with ice water. The negatives can take 10 minutes to develop, but it can also knock the contrast down so far that you can see clouds in the sky along with shadow details. When I do that I extend the presoak and really make an effort to have it hit the developer all at once.
I process my 11x14 paper negatives in a tray ( plastic for storing paper from the office supply store, with a lid ) and my 8x10 in a plastic pitcher that has a good sealing lid. With both of these it took some practice so learn how to get the developer to roll over the negative smoothly and evenly and quickly.