You can check the solution requirements - just put 500ml in the tank and measure the depth with a ruler., and compare it with the film dimensions. Usually insufficient chemistry or a slipped sheet leaves a band across the film. Localized marks can come from residual chemistry on the film before the developer (not with clean, dry tanks). Sometimes touching the film surface with sweaty finders can delay take up of developer. Work out how the sheets were in the tank. That helps decide if the processing was at fault.
If you use a changing bag, be wary of pinholes or a luminous watch. I've seen both leave marks on film.
Cleaning holders - I find that the nozzle from the vacuum cleaner works well, possibly with a soft brush first if they are really messy. Don't forget to remove the dark slides and do those and the light trap. Then keep them in bags all the time except when loading/unloading, or exposing. Plain old zip-lock style bags work, though some people in dry climates prefer anti-static bags. If you have to wash a holder, don't get the felt trap wet. Use a Q-tip or damp cloth.
When I empty a holder I make sure to put the slide in in the 'unexposed' direction (white outside for me), and leave the loading flaps open, then put it in the bag. That gives me three options - empty, loaded, and loaded-exposed.
