hi there, i had this happen also, isnt it a nightmare, i was told that they wake up because they use the dummy as an aid to go to sleep, then once they are asleep they relax and the dummy falls out, then they stir and wake up and go to suck to go back to sleep and the dummy is gone, if you dont want to keep getting up every 5 min then i would get rid of the dummy,i did and it saved my sanity.
Good luck.