Loose valves can happen on trilams, but it's a given on neoprene drysuits as the material compresses a bit with initial use, so they need to be snugged up every few dives until the suit material is fully compressed.
Any kind of squeeze at the valve itself sets up the negative pressure gradient needed to get water to leak in past the valve, whether it's something on the seat like salt, sand, a scratch, etc or a leak between the valve and flange itself.
With a couple valve and undergarment combinations, the undergarment can inhibit venting the suit, leading to the diver trying to manually vent more gas from the suit, and in any case with any valve/under garment combination if the diver continues to try to vent the suit by pressing the valve open manually after the suit has dumped all the gas near the valve, it will let water back in.
In one particular case I had a valve flange in the suit that leaked under the flange, but that's pretty rare.