is it the female or the male that is in the plant? (to tell the sex look at the anal fins..on the female its a normal fin,on the male it looks like a cone (its called a gonopodium)
What else is in the tank and what size is it?
Some baby fish will be eaten when born but some will manage to swim into ur plants and ornaments where they will hide and scavange for a few weeks until they are large enough to be safe (so dont worry, u will bring platys up to full maturity with very little input)
My platys were VERY VERY shy when i added them,its taken about 7 weeks for them to become properly comfortable in the tank and when they feel threatened and stressed (or due to drop babies) they will just sit on their own in a secluded area.. this could be what is happening with yours..does she look unevenly bloated? any problems with any fins or scales? breathing ok? is she sat ON the plant/substrate or is she keeping herself up by swimming?