A stop bath will prolong the life time of your blix in the Nova, but it is not necessary. Do you have fresh RA-4 paper and stocked in the right way (fridge <12 degerees C).

Are there signal lights (dokatimer) or other equipment which can give a problem on your RA-4 paper. Specially Kodak Endura is very sensitive for these kind of failures.

Best regards,