This happens sometimes when traffic is just to high. I had to wait (at the mccafidiot tweet time) for 10h for withdraw from cryptopia to get some conformations.
Hadn't seen it with ETN. What I found strange is that the backlog was about 40minutes but it still meant several hours before my transactions were in the blockchain. I tried with different levels too and same result. Weird, but I guess miners choose a random set of transaction and not always yours is included in it.