Ivan IS the tech. I see no way that a valve can stick once it is broken in - UNLESS there is dirt, damage or distortion as when you play, there is a continuing wear. If a valve is so worn that the oil no longer provides a viscous shield, it can also stick. I consider that damage. Dirt is DIY. The magic marker will point you in the right direction for the rest. You can also look for scratches in the valve casing. Putting a valve in a different horn tells you nothing.