Just reading this right after the even more recent topic about ollies, are you too bent over? You said you looked slouched over, and if you look at this video that was posted in the aforementioned thread the last point might hit home.
You gotta leave yourself some space to actually jump, the board wont raise you in the air with just pop alone. Think about how you would jump without a skateboard, are you slouched over then too?
In an old thread I recently read someone mentioned kind of sitting back when going to pop to get the posture more upright, I have been working on this now since I feel it will make a big difference on all the tricks as it affects how you align with the board when landing as well.