Thanks for the solid advice, gentlemen!
I pulled that pic off the net to give you an idea of the position that troubles me. I do not have a spectacular visual memory, and it's difficult for me to describe. I'll give it a shot:
My opponent has his top arm controlling my neck (with his elbow driving into it) and using his bottom arm to push down on my knee. This makes getting a butterfly hook difficult because he has control of my leg. My top arm is usually trapped into his body and the pressure he's applying (plus his angle) makes it difficult to establish an underhook.
Against less skilled or lighter opponents, I simply sweep them over to the opposite side. Unfortunately, much larger individuals can hold me there and pry open my open guard which irritates me to no end. I hope this makes more sense.
In the meantime, I will take the excellent advice of seeking proper escape routes leading to more beneficial bottom positions.