I have experienced this problem as well, if the market volume was too small for a large order.
Your default 4m order is not a small volume.
Try to allow more slippage for your orders, you set slippage to 0 only! Try higher pips, such as 1.0!
Try to use place bid/offer orders instead.
https://www.dukascopy.com/wiki/#Place_BID_OFFER You will never get a worse price, but you will probably get more partial fills on your desired price.