I've found the same thing; numerous factors go into muscle gain. For me, I no longer eat as much or as balanced as I did when I was training seriously, which means I don't make the gains I used to. I also lack a partner which means no spotter; I think spotting is a huge help. I'll be surprised if I am back to my old level even within 6 months, which is a really long time just to double the weight I can put up in most exercises.

But you know what? I don't really care. Just the act of going, of noticing slightly flabby parts of me start to iron out after only a month... I'm pretty happy with it. It still makes me sigh, though, when I can't even bench 145.