I've gone YEARS cycling through the same weights on assistance exercises at times. The thing you gotta keep in mind is that that's all they're for: assistance. As long as the main lift is going up, the assistance work is working. If your assistance work keeps climbing and your main lift doesn't, you're training wrong.
Additionally, consider this: not increasing assistance work while increasing the primary lift IS progressing on assistance work. Being able to maintain the same reps/weight on a movement while having hit a heavier/harder movement first is most definitely a form of progress.
Let take an obvious example: Let's say that you run 1 mile, rest 3 minutes, and then deadlit 300lbs for a single. The next time you train, you run 2 miles, rest 3 minute, and can still deadlift 300lbs for a single. Even though the weight didn't change, the amount of fatigue you had before the lift DID change. You were able to lift the same amount of weight in a more fatigued state, which is to say that, in a less fatigued state, you could lift even more weight.
Thus, if your first lift of the day keeps increasing and you're also able to still hit your previous lifts on your assistance work, everything is getting stronger.
And of course, this says nothing of the various other methods of progression that don't directly relate to weight lifted (shorter rest times, faster rep speed, better control/mastery over the weight, increased movement efficiency, etc).