I used Poloniex once without any issues, but I didn’t like it because of poor customer support. It didn’t fit me also due to the confusing interface. Currently, I stick to Cex (https://cex.io) and I never had any problem neither with their customer support nor with authorization. The exchange has appropriate certificates and Fincen registration that speaks for its legality. So, maybe you should switch from Poloniex to another platform like I did?