when do you deload vs keep trying same reps and sets?

[Mike Hunt]

So say I am doing pushups and I want to do 3 sets of 6.  Last week I was doing 6 sets of 3 and it was easy. This week is hard and I sometimes can only do a set of 4.  So does that mean next week I should do the same 3 sets of 6? or should I follow my plan of going back to 6 sets of 3 (sort of a deload)?  thanks