Can my TRVs be saved?

I found various makes of TRVs had been fitted to the radiators by the previous owners. One make in particular, Westherm four, all have the same problem with the pins. The pins aren’t stuck, they move freely up and down when using a pair of grips but they stay in the up or down position and don’t pop up under spring pressure. I’ve banged them a bit and sprayed them with WD40 several times over several days but the pins still won’t spring up from the down position.
I guess they are faulty but can they be saved somehow or is it time to call out the plumber to have them all changed?

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