I think the problem is that they use a wedge shaped metal shutter (a bit like a sluice gate) thats forced into a shaped metal groove to shut the water off. It only needs the tiniest bit of scale in the groove or any other crud to stop the shutter closing all the way.
A proper stopcock uses a rubber/metal interface and doesn't suffer from this problem.
Tim
If used part open (or even fully open in soft corrosive water areas) the gate can erode either due top cavitation or corrosion, sometimes resulting in full flow, otherwise a failure to cut off completely.
I've given up using gate valves.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.