That's because the moisture in the air diminishes the potency. What I would do when we had bird feeders was mix with with petroleum jelly, then smear the jelly where the squirrels climb. Stays potent longer. Oh, and be sure to use gloves when you do that.
Dan
Gloves, petroleum jelly, and a bird feeder. I saw a video like that once.
(It was about U.K. residents trying to rid themselves of gray squirrels...now admit it, who had a dirty mind?)