Maybe you are not warmed up properly. warmups are typically 30-50% of your max, lots of people do a 20 rep with just a 20kg bar, then a light weighted set, ~30%max and a moderate weight set ~50-60%max.
Work sets should start around 70%max, 6-8 reps, unless you are focusing on pushing up to a new PB or 1RM, which you shouldnt be doing all the time..
maybe you body needs to get used to the heavier weights more gradually
There are heaps of training options and numerous ways of gradually increasing your weights and reps each session, just dont fall into teh trap of going too heavy too quick and sacrificeing the volume in your training, target about 30 reps each session. not includin warmups.
I have also noticed that on subsequent sets you can push out an extra rep, the time betwen sets will effect your performance as will your energy levels, ie what you have eaten prior to that session, and how much sleep you have had.