Semi stand in larger tank

Recently I was getting uneven results while developing FP4 in Rodinal 1+100 1h with no agitation. It used to work well with old orwo film. I'm not sure what are the causes. I'm going to do semi stand this time - a few inversions at the beging and one or two at 30 minutes as from what I read it works even for other people.

I have 4 or 5 rolls which I plan to develop. I used to develop one roll at a time as I had small tank. Now I have bigger one - 6 x120 or 5 x135. Is it safe to develop them at the same time with the agitation above? I'm bit afraid that higher tank can exaggerate problems with uneven development.

I've had good results with 120 size FP4+ at EI 100 in Rodinal 1+ 100 for 15 minutes, several agitations in the first minute, then one agitation each 3 minutes after that. My tank holds 6x120 or 8x135 films. There have been no problems using it with many films at once. It always amazes me - how active Rodinal is at these high dilutions. To me, FP4+ in Rodinal is perfect - the negatives have always been easy to print.

My tests with stand development gave unevenly developed negatives.

I do Rodinal 1:100 for an hour and have never experienced any uneven development. My agitation method is one 15 second inversion at the start and another at 30 minutes. Works like a champ. Regarding tanks - I always double the developer volume to film ratio. In other words, 2 rolls in a 4 roll rank, 3 rolls in a 6 roll tank, etc... Because of the dilution rates, I've found this to be necessary not only with roll film, but also with the 8x10 sheets I develop using semi stand. My results with a shorter tube resulted in high base fog at the top of the film - a taller tube (more developer volume) will eliminate this. Good luck!