I think its probably down to your strength endurance (if there is such a thing) giving up on you.
I would keep the same weight but do less reps eg,
1st set 10 reps
2nd set 8-9 reps
3rd set 6-7 reps
or whatever you can manage.
over time you will be able to do 10 reps each set, when you can do that put the weight up a bit, then start all over again....