When this happens to me, I just lower the weight and try to make it feel heavy.
For instance, I can leg press about 360kgs. But I was off last week. So I loaded up 240kgs and concentrated on my form, really drawing out the eccentric, pausing at the bottom and squeezing to come up.
I used a safer weight, still got a great session in.
So try just going to your capabilities. You can't be strong every day.