Most likely, there are a combination of factors here. First is the tank size--with fish that get big, starting them in small tanks can be a disservice to the fish. The smaller tanks, without frequent water changes, accumulate the hormones that the fish put out--and this reduces their growth. With a larger pleco in there, this guy is getting a double hit, and will be slow to grow--the larger ones growth is likely also slowed.
Since the commons are not appropriate for a 20 gallon tank, I'd move them to something bigger. I had a chocolate that was in a 40--weekly water changes, but several larger ancistruc plecos in there with him. He barely grew to 6 inches in a year. Moved him to a 55, with no other plecos, and he doubled within a 6 months.