One of the worst things that can happen to anyone learning to play guitar is that they spend too long on one thing, then start to lose interest, then quit altogether.

By all means go on to the next lesson and learn something new (or a new variation on something). You can always go back to earlier lessons and practice that until you get it up to speed.

I sometimes go back to earlier lessons just to remind myself of some of the details I might have forgotten or simply for the enjoyment of playing the practice piece at full tempo.

Little tip: always try to end "learning time" playing something you play well. It leaves you in a positive state of mind, and sometimes reinforces just how far you've come since first picking up a guitar with the intention of learning how to play it.