Thanks for your reply, but i think that is global problem not only for me. But I would like to comfirm that from another users :-)
I haven't heard of this global problem about ETC so I'm not really sure
but If there really is one, then I would have known it considering that it's a large scale problem since you've mentioned it's "global"
Just to clarify, We're talking about deposits here right? and not withdrawals? because depositing is just as simple as sending an X amount of coins to your deposit address which you've got from X exchange, and after the transaction "confirms" the amount should be credited in your account on that exchange or your wallet there to be precise.
Check the transaction hash of your deposit in any ETC block explorer if it is confirmed or not because if it is, poloniex doesn't have any reason not to credit your account.