doing some primitive research whilst playing the mandolin I have worked out that the A string is the one that gets played the most. Given that the zouk is virtually the same but an octave lower I guess your A string gets treated the same. Added to the fact thats is an unwound, ie smooth, then I bet its not as well tied as it should be to the machne peg stile. Why not try a different way of tying the string on. Don't take this wrongly or disrepectfully but players don't always know how to tie a string properly, they've never learned.

By the way I am pretty sure that each course of strings on an instrument like this is at a different tension so it could be that one way of tying a string might work with a string of lesser tension and might not work with a higher tension.

stevethesqueeze