You are probably going too deep down wind which is why your sail is losing power. Head up wind a few degrees to regain power.
The sail is raked back as you go down wind. When heading downwind the apparent wind is at a much different angle then when on a beam reach or sailing up wind. As you increase your speed on a beam reach the apparent wind moves more from your side to the front of the board. This also occurs when you head up wind. But the deeper downwind you go the more your apparent wind starts to move back toward your side as it slows down.
Staying sheeted in or slightly out has much to do with your outhaul. If you have an adjustable outhaul or you have set your outhaul to minimum, then stay sheeted in as that will trim your sail best to the slower apparent wind. If your outhaul is set at max then you will have to sheet out slightly to try to match your sail to the slower apparent wind. If you sheet in a flat sail going deep down wind you will stall the sail.