For your original question, let us know more about your situation... like what mod, and type of button and button placement... someone will be along to help out... some of the things I have encountered with mech mods might be helpful in the meantime...
First - if it's a clone, tolerances could just simply be sloppy, and the parts work poorly together. Some are better than others, but the good "original" mods are usually made with more pride and craftsmanship. (please excuse the generalization, I know there are many happy clone users out there too)
but
Second - as was mentioned - our hobby is sticky... literally... so oftentimes all that is needed is a good cleaning. The beauty of so many mechs is that you can't really break them (at least not worry about electronics) while cleaning, and most are easy to disassemble and spot the gunk that's giving issues.
Third - you might be encountering a mechanical failure... specifically, if you have a spring under your firing button that is damaged, weakened over time, or might have shifted and kinked, or something. Sometimes harder to get to, and not easy to spot from the outside, this one can be more frustrating, but possible. Replacement springs/spacers/etc might be tough to specifically source, but someone had to buy them or make them for your mod in the first place, so suitable replacements or even upgrades (stiffer, softer) for your tastes could be available online, or at a local hardware store. I know... a little more work, but you get to decide what's worth it, depending on how much you love your mod.
You could also always use the situation as an excuse to feed your shinyitis
Welcome aboard.