It could be that the d-pad traces on the circuit board have worn down over time and now the conductive material on the membrane isn’t able to make a good connection. You could try and check the traces with the continuity setting on a multimeter if you have one. Maybe very carefully going over the traces with conductive paint could help?, but I have never done that for a d-pad before and don’t know if it would work well or hold up over time.
It could be that the d-pad traces on the circuit board have worn down over time and now the conductive material on the membrane isn’t able to make a good connection. You could try and check the traces with the continuity setting on a multimeter if you have one. Maybe very carefully going over the traces with conductive paint could help?, but I have never done that for a d-pad before and don’t know if it would work well or hold up over time.