I can shock the chest muscles with supersets and hitting different angles using cables, weights and push-ups in supersets, but I can't make it burn anymore during the workout.

I do varying inclines on the Smith machine, flyes, incline hammer presses, crossovers, etc., but I can't seem to recapture that beautiful burn I get with ab and arm workouts. I work to failure, and I'm using moderate weights, high reps (12-15), several sets (5-8) and holding briefly on adduction. My chest is getting big, but there is no burn anymore. Any ideas?