Suz wrote:

My question is why does the solder not work? Is it conductivity of that
type of solder? Is there an easy solution?

from my past experiences of trying to solder low voltage or thin
stranded wires on flexible things (like speaker wires & stuff) what
happens is..... you solder the wires without problem, but create a stiff
un-flexible joint where solder ends and wires start.

Unless this point is adequately supported past the connection, after
just a few wiggles of the wire it re-fractures losing connection.

Solder it's self is a good conductor.

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