Hey, i have this problem too and am about to open it up. Its likely the small spring in the button has become detached or broken.
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Edit: Taken it apart in about 5 minutes, spring used to puch back against your finegrs was snapped. Replacement one bought on ebay for £2. Will post if it fixes it.
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In the mean time reversing the spring position has given me back maybe half of the resistance so a decent enough temporary fix.