The problem may be .. if not working at all .. the close-off seals are worn/perished/stuck. Sticking happens through period of non-use. In the summer, it's best to open ALL TRV's fully, so they don't stick shut.
Honeywell/Drayton TRV2 and 4's are dear because they use a large lisuid-filled capsule instead of a waxstat type. Many prefer them for looks (chrome cap) or the fact that they are more sensitive in operation .. therefore control temp a little different.
However, many tradesmen use a cheap standard TRV .. because they never whistle . .. and they are reliable.
Take your pick.
Valve replacement means drain-down.