Yes, sometimes, it is like it pools in the cup and capillary action causes fluid to wind up in the air hole and the plug nipple. I take the whistle tip off and still holding it upright, get my head under it and blow up into it from the thread end. That fixes it for me. So assuming your issue is the same as mine, it may help you.
I always have a few sitting around. Sometimes for weeks. I'm finicky with my juices. When I get juice in the nipple, I just suck it out with a syringe and it's good to go, usually. Doesn't happen too often for me, tho.