Why can't I gain weight faster?

You sound like you have an endomorph body type (it puts you in the hard gainer category) and though I have not experienced your weight program I'm gathering your doing 3 sets 8-10 reps? If so, this is strength training and you will not gain much. You need to be doing 4-5 sets 6 reps to gain mass (if your bowflex can give you enough resistance for 6 reps then your good to go, if not, head to the gym - with a spotter)