Yea, usually the price drops before it is filled
That's the risk you take with limit orders. It seems like you are waiting too long before you sell. E.g. the market is already dumping
and then you put limit sells on the book above the spread. Meanwhile, the spread is dropping because the market is dumping.
Generally with limit asks, I am selling into strength. I'm selling when I've made a good profit, hit my target, and others are buying. If you wait until the market has already turned, then only small fish buyers will be left market buying, and chances are your orders won't be hit.
There's a pretty hefty charge for market orders. I sell at the last available price, which should be the fastest. If I sold at lower then it would take longer as I would have to wait for the price to drop to sell.
Yeah, GDAX market order fees are brutal. All the more reason why you should sell into strength with limit orders and reserve market orders for when you need to immediately exit the market.