I've been thinking about this same problem. Seems a lot of people have problems gaining much on a second/ or "upgraded" cycle. I know this is what everyone is saying, but want to throw in my version....
If you gain 10-15 lbs on a good cycle and then attempt another 3 months later, you run into the problem that you are now heavier with more lean mass. Which might mean that your metabolism is higher and you are just going to need more calories to maintain new mass in the first place. So, you have to adjust your calorie intake accordingly.
Plus, though steroids help with recovery, as your strength and intensity go up, so does your need for recovery time. If you are training hard enough, you are putting out much more effort than before and need at least some extra rest, steroids or no.
I'm quickly re-learning that you can't do the same thing over and over and expect different results!
Jacob