Why is the lock button stop working sometimes?

Hi,

Liquid damaged devices, are very unpredictable, this is because, there could be many damaged components.

You would need to inspect the entire logic board, and see if there is any damage, such as corrosion, or knocked off chips, clean up the logic board with either an ultrasonic cleaner, or you could go the DIY route, and clean it with a toothbrush and 99.9% isopropyl alcohol.

Even then, liquid damaged devices shouldn't be fixed for the sake of the device, as problems can reoccur later in the devices lifetime.