As in the
juice isn't getting sucked back down?
Too much cotton can affect your
juice flow. If your cotton is blocking the squonk hole or the path it would take to get back down into the bottle, coming up could be no problem but going back down could prove difficult.
If this isn't the case, a pocket (or purse) kept Reo can gather tiny particles of dust or lint in the chamber. If you squonk while some of that stuff is resting on the ceramic, it will get sucked down, and collectively cause clogging. Try taking a piece of thicker wire, like 26G or twisted 28G, and cramming down the 510 connection (with atomizer removed). I had a similar problem a couple of months ago and ended up pushing what I still believe to be a dead bug
through my feeding tube.
Edit: Just realized you meant your catch cup. This happens to me frequently. I just removed my atomizer and clean both the cup and the bottom of the atty with a Q-tip and some rubbing alcohol. Works for me. Good luck.