Dear Eitan. If this is PtPd printing, mostly I’ve found it is caused by humidity variability in the paper. These have to be controlled incredibly tightly.
The other variable: We use a P9000 with K5 Pro ink for our negs. We make sure to agitate the cartridges (700mL internal bag carts) so there is NO settled ink visible at the bottom of the cart. We do this every Monday or before the print-session if longer time has elapsed. Also I do a regularly head cleaning on all channels prior to the negative print session just to ensure all ink in the nozzles is fresh.
regards
Walker