well, are you getting fat? if you have not gained noticable fat after adding 20lbs to your frame, then you're just not seeing the muscle.
this is why photographs are important. noticing a change over 4 months is hard. comparing before and after photos taken 4 months apart makes it easy to see change.
that or do a 'photo a day' project and then time lapse it into a video.
body fat tests also help you gauge how much weight gain is fat vs. muscle. its hard to bulk without adding some fat, but you can definitely maximize muscle gain while limiting fat gain to a reasonable amount.