Did you send an email to support concerning this issue? If no then try sending a mail to them so that they can check and tell you why it is that way.
I emailed support, waiting to hear back. It's happened twice now, I check the transaction on the blockchain and the fee is too low, right about 500 satoshis. The first time, support said this is a highly unusual occurrence as perhaps there was a dramatic dip in mining fees at the time of my withdrawal (and I got reimbursed, to be recouped when the transaction eventually clears), but because it happened again on my second most recent withdrawal attempt, it's looking like that might not be the reason, it's curious. I've used the same BTC address with no problem like this in the past.
I checked the last withdrawal from my account before these recent two (it was months ago, and it was under 0.0002 BTC) and the fee was just under 16,000 satoshis, so there was no issue then.