Is it normal for orders to take a while to fill even when the mark has gone past my limit price?

Your orders probably didn't get filled because the Mark price isn't a real price where buyers are actually willing to buy from you. It's the AVERAGE of the bid/ask prices, which for some tickers (like UNG) tend to be 10 to 30 cents wide. It's normally to have to wait a while for those orders to get filled, compared to tickers like SPY and TLT where the bid/ask spreads are much tighter (and therefore the Mark is a more accurate representation).