Hi Mike,
I had this once, years ago, I had always used the little post to turn the film in the tank and got the same difference in density at the edges. However afterwards I always sealed up the tank and inverted it for agitation. I have been devving with rodinal using Lucky 100 film (which I won't be using again!) but 12 mins at 70 degrees, 1:50, agitate for 1 min inverting, then leave alone for 4 mins, agitate for 15 sec inverting, leave for 4 mins etc. Works a treat.
Ben