What are your rest periods between sets? I know that when I’m trying to add size (strength), I give myself 2 - 3 minutes between my heavy sets (and do very few sets), and sometimes I can even get more reps on later sets (as I get more focused I guess?).
How many reps are you doing in each set? If you’re like me, maybe you’re doing too many. I’m finding that singles work best for me. I remember when I was doing sets of mmore than 5 I wouldn’t be able to do more than one set with a certain weight, but I can do up to 15 singles.