im not too sure but, when i watched a youtube video before of a controller being modded, i remember the guy saying that there is some varnish over the soldering points which you need to scratch off otherwise your solder might not stick, im not sure if thats correct tho.

heres the video i watched.

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Use flux that would be the most obvious point out.
Also the tip might be oxidised or you didn't apply the solder correctly.
Make sure to heat up the solder then try to add more to the bottom, don't scrape the solder off the iron on to the board by applying the solder to the tip then trying to cover the point.

put it this way, there was a little gold circle around the holes like you see on the ones next to it and since mine has come off, the solder is not sticking.

Does your left trigger still work when you pull it? If the pad has come off than it shouldn't work

GEN3RAL_ALDEEN wrote Does your left trigger still work when you pull it? If the pad has come off than it shouldn't work

try that and if it still works, try heat the point that you want to solder with the soldering iron and then apply the solder, that should help the solder bond to it.