Most organ bellows normally have springs on them so they deflate when no air is being blown into it. I play on a small (very small - 10 stops in total!!) and you can get inside it easily and the bellows do have springs around the casing so it deflates, but there are also weights on top too. The weights are there so the bellows provide enough pressure into the system so most of the time, there should be a constant pressure! My organ has then same problem as yours - Majority of stops drawn and a nice, full chord and then, the pitch goes out and goes soft. This would be because of an air leak in the system (normally known by a whistling noise) so, check the wind chests ect. If there is a leak, contact your organ builder. If it's the blower not providing enough air, also contact your organ builder. Feel free to ask questions if needed!