Bumping up volume on assistance work.

Hi everyone,I would like to know if anyone knows how and when to increase volume on assistance work. I do a certain exercise for about 4 to 5 weeks and then rotate it so I do not adapt. When I switch to another exercise I feel like I have not made any progress since I am doing the same weight,sets, and reps for those 4 to 5 weeks. When I rotate them it feels like I'm starting all over again and not making any gains. How do I know when to up those lifts, whether it be in sets, reps, or weight?If anyone has any suggestions I'm all ears...or eyes.

Example:

*Weeks 1-4/ 5 sets-12 reps w/60lbs on dumbbell rows

*Weeks 5-8/ 5 sets-12 reps w/100-110 lbs on lat pulldown machine.