A photo can help us see what's going on, but, my suspicion is that the cleat has bowed to the curvature of your shoe. You can't engage if there is any deflection of the cleat.

There are shims that should be installed under the front and rear of the cleat; when you look at the shoe from the side there shouldn't be a gap between the bottom of the mounting plate and the sole of the shoe.

http://speedplay.com/index.cfm?fusea...ctionfaqs#na58