Depending on whether you consider it "cheating" or not, you should find out if there are other caves nearby that are consuming your mob cap.
If you press F3 in game to get the debug menu and look at the value for E on the second row. This will tell you the number of entities (including mobs) that are visible/in-game. If the second number stays high when all the mobs in your trap have died, then it is likely they are spawning elsewhere.
Spawn region is roughly 272 x 272 around the player, with a 24 block sphere (yes they apparently do exist in MC) around the player excluded from possible spawning. This means if there are any dark spots in that region, mobs will amass there and your mob trap will die out.
If you want to take the "cheating" a little further, you can use something like minutor to show you the caves in your map and head out to light them all up. You'll also need to light up the ground in that 272 x 272 region as well.
This is why building a high rate mob trap "legit" is a pain in the ass! I'm amassing TNT to destroy the entire 272 x 272 area down to bedrock, that should stop them spawning :)
NB: I use the word "cheating" lightly, it's your game, play it how you want.