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Push button switches can become unreliable with use. Beneath the external button there is often a standard tactile switch soldered to the circuit board. Sometimes a simple solution is to insert some packing, such as a folded piece of paper or card, between the external button and the tactile switch, allowing a little more pressure to be applied. Replacements are available very cheaply on eBay. Amongst several standard sizes 6x6mm is common, available in a range of heights. As a short term measure you may be able to swap an unreliable switch with another on the same circuit board operating a function that is rarely if ever needed.