FF is a water repellant. Once the fly gets eaten, the fly get water logged and you need to get the water out of the fly before reapplying FF.

I use a traditional floatant like Aquel that soaks into the body of traditional flies, and I use Dry Magic on CDC. Then apply the FF. After the fly gets taken, I squeeze the water out with an artificial chamois, then I treat it with a desiccant FF combo like Shimazaki Dry Shake or Top Ride. The combination products will dry out the fly and coat the fly with the same material as FF. Blue Ribbon Floatant, that Loon Outdoors sells as re-filler for Top Ride, is the same material as FF.

Floatants work by 2 methods, soaking into the spaces between dubbing and/or coating the fibers and feathers. FF cannot soak into the spaces between dubbing like gel floatants can. So use both methods.

A third method is to "presoak" the flies in water repellants such as RainX or the old style (green can) Scotchguard. Spray the Scotchguard into a small capped bottle and soak the flies after you tie them, then place on a paper towel. Recap the bottle after use to save the Scotchguard.