Pat,
Is it possible that you're slowing down a bit?image



Sorry but i had to say it.
Another "trick" with spring bobbers is to pre load the bobber till it is 1/2 way compressed with the weight of the bait and hook. Keep checking the bobbers distance off the rod, if it reduces or increases, set the hook. I use a slow jigging technique and the bobber stays about the same difference till a fish messes with it.
Hitting on the drop of the motion is a crappie trait that has shown up often. When the spring raises, set the hook now!
Another thing I've noticed, when this techinique works once it will ususally keep getting fish for a while.
Bluegill have this propensity also but on their bite they usually put a curl in your line as you are lowering the lure.
Just another "old timer" tidbit.