Is it a Syzygium or a Eugenia - and what sort?

Most of them are 'full sun or part shade' plants so some sunlight each day would be an idea. Start with morning sun - somewhere it won't get too hot because it won't be hardened off at all. Have you got a porch you could put it on? Or under a garden tree that lets dappled light through.

You might be overwatering, too. If it is drying out really quickly between waterings - it might need repotting - and possibly some root pruning if it has run wild in the pot. If it's OK, then put it up on feet in the protection saucer so the surplus water can drain away and air can get to the roots. Wait until the soil feels dry as deep down as the first joint on your finger - then water thoroughly. If the pot is not too big - put it into a bucket of water and wait for the bubbles to stop - but get it out within ten minutes, and let it drain before putting it back on its feet.

And some of it could also be 'changing leaves' for spring. A lot of the evergreens do a leafdrop around now.

Another possible is being in a cold draught. Easy-fix, that one!