A couple of things come to mind, you might try setting under additional cloth collision options
object vertex against against cloth polygon and polygon against polygon. If you have a coarse mesh and the sim is just seeing vertex against vertex you can get poke through. Also I find sometimes if I have been trying several sims I need to delete all the frames (change frame setting to 1 of 1) then go back to 1 of 30 or however frames your running and reset your pose and hit the clear simulation button. If you have been running several trial sims just starting a new one over doesnt seem to clear out all the info from the previous sim. One other thing make sure there is no contact between the cloth and the object at frame 1.