Is the Platy a male or female? Remember that it does take time for the fish to settle and in many cases the fish can hide for weeks or even months on end before appearing. Back in my days of marine fishkeeping I used to keep a few species of Pseudocheilinus wrasses. Most of them would hide for around 2-3 months before jumping out of the tank.

It is also quite likely that the fish in question is being bullied, Platies are normally very peaceful fish however the males can quarell betweeen themselves, and older males can make life misery for younger and weaker specimens. Can you see any spots, lumps, wounds, cotton wool-like growths or anything unusual on the fish?