Since Kraken is utilizing the omni protocol for Tether, I asume this would be a correct transfer. Could I be wrong?
Since you can deposit tether to kraken, this shouldn't be an issue.
But this implies that you have used a USDT deposit address (not a BTC deposit address). Can you verify that you have used the correct address?
Does poloniex say the transaction has been successfully sent? Did you get a transaction ID ? If so, check it on a block explorer (
https://omniexplorer.info/).
Depending on the status of your transaction, you'll have to contact either poloniex or kraken.
If the transaction has been sent and confirmed successfully: Contact Kraken.
If the TX has NOT been sent, contact Poloniex.