Just to go slightly off topic here... but you need to understand that a "spotters" role is to prevent you from getting squashed under a bar.
his/her job is not to help you do the last 4 reps of a set while stroking your ego by yelling "All you bro".
if you are getting 8, 6, 4 reps at 60 and the last set is being assisted for 4 reps, it's useless (as can bee seen by the no increase in the last 6 weeks).
Try some of the suggestions (higher reps with less weight, less reps with higher weight)
But it might also be something simple like not enough rest between sets.
in a commercial gym it's a bit hard to sit around for 3 minutes between sets but sometimes you need to.
Is Bench press the first exercise for the night? if not, try doing it first.
finally, weak bench can mean weak triceps and or delts.
bench press is as much tricep and delts as it is chest.
How much can you press over head?
can you do bodyweight dip? if so, how many?
how much weight can you lift with close grip bench press etc.