Trident, Roel is correct. The vast majority of the switches are sealed units with carbon connections. Pressing them hard wears them out quickly--they were never intended for this. Depending on the brand of keyboard, you may be able to replace them, but this will require a major dissassembly and a significant degree of electronic expertise because most are usually part of a major printed circuit board and micro-soldered.
Good Luck,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)