"If it dries it dies" always store them wet.
You could put a single drop of dish detergent in 500ml hot water and scrub the probes with a soft toothbrush. Rinse well. SOAK in probe storage solution overnight then calibrate it.
If that doesn't fix it it will need replacement.
I've gone back to liquid drops. Not super accurate but close enough.
Hang on, just remembered that weak batteries could be the problem.