Puzzling problem

I’m running a $400 CBO with 3 ad sets and 22 ads. One ad set is performing very well. The other two had a couple of “spike days” with 3–4 purchases, but after that they dropped to almost zero for 3–4 days — no ATC, no purchases — even though CPM is cheap and CTR is solid. Why does this happen? And should I keep those 2 weak ad sets running, or turn them off and push all the budget into the strong ad set? The good one also hasn’t exited the learning phase yet because it only gets around 28 conversions/week at best (not close to 50).