just an idea from me
what about to make more free levels before you must buy the game (it costs u nothin just time)
to get the user more addicted to your game
becouse the free trials plays a lil bit short (to short) in my opinion
but dunno why your game dont have so much downloads
becouse i find its a nice one
maybe some free worlds + add some little ads
and if the user wants to play more worlds he can buy more worlds
but this are just some ideas from me
i have no experience in mobile games or anything so ^^