root gate in they have to open it theirself or the set gate that releases the trigger when their back drags it... You tried tying it open and feeding it for a while?
You can also weld you a pin on the side of the door then hang a Ubolt with a Lhook rod that will set at a 45 angle while holding the pin. When the hogs back hits the gate the Lhook will fall back strait and the door will shut when the pig is all the way through.