Yeah, the two things I would look for is.
1. Is it the accurate size cylinder for you? You can Check Crazyed's guide for this, its sticky in the pumping section. You are a big dude so yeah 12" length is good, but you also need to worry about the girth. Is the tube too thin or too wide for you?
2. Does the pump have a gauge so you can see what pressure you are pumping at.
If these two dont checkout good, you probably are going to have a hard time with it.
EDIT: I found the pump you are talking about, Ive seen it before. It comes with a 2.5" cylinder and the pump does not have a gauge. So according to Crazyed's guide a 2.5" cylinder is good up to 7.75" girth, so you need to measure your dick with a measuring tape around and see if you are around that girth. Too small no good, exact or over that no good either.(highly doubt you are over that) And you just need a better pump, no way around it, get a vacuum brake bleeder pump with a gauge.