Durgo valve

[jzhou]

My plumer cut my vent pipe, which was go through the roof, and fitted a durgo valve next to the toilet (in a quite high level).

When I flush the toilet, I used a torch looked inside the Dorgo and noticed there is no movement of the valve (no air get in) but the toilet flushed well and no smell out of bath and sink. The only difference is that the water level of toilet trap is lower than before. Is there anything I need worry about? Why the Dorgo doesn't open?