Does the pressure go very high when the central heating is on? If so, the problem is the expansion vessel, not the pressure relief valve (which is just doing what it should).
Otherwise, it may be that the valve has got some muck stuck on the valve seat. You shouldn't operate the PRV by hand as it can cause this to happen. However, once it has happened, the only way you might clear it is to try flushing the muck out of the valve by operating it by hand. That may fix the problem, or you might end up with more muck -- worth trying a few times at least if necessary, before replacing the valve.
Once fixed, you will probably need to top up the system with inhibitor if you have been losing water from it. It might be that the system needs flushing out first if it has significant debris floating around in it, which could in turn be due to not having enough inhibitor in it.
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