Well you are dieting arent you? Hmm maybe your cals are too low? are you dropping weight quickly? The other thing is if your a runner/cyclist you may be overdoing it a bit too if you are training hard at that aswell.
The only other thing I can suggest is to play around with the rep ranges a bit too, maybe try some triples for a while and by the time that stalls the weights you were failing on at 3x5 will be easy.Just my .02c