Lately most of the exchanges won't reply to you for ages, a consequence of the tsunami of new users whi have been joining all of them. Some exchanges have hundreds of thousands of customers by now and they didn't scale up with their support. And Bittrex has become one of the biggest lately.
Yes I think you are right because they are busy on managing their server and databases that's why they cannot asses all the reports of customer. But no regrets so far because I know my funds are very safe on Bittrex.