baby sleep

Natalie
Having a really hard time over here!! Baby has been sleeping in her rnp sleeper since birth ... she's almost too long for is so we're trying to transition her to the crib but she will not stay asleep on her back! Wakes up screaming - she does have reflux and is on zantac. I tried elevating the mattress with no luck. 
It figures she sleeps great on her tummy 😩😩 anyone else having issues? She is 14 weeks