cleaning the top of the sensor sheet does nothing. but make it shine. the sensor sheet is completely sealed and the contacts cannot be cleaned. you could have big chunks of weed in between your pads and the sensor sheet and it will still work. there may be a connection issue at the terminal where the ribbon goes into the board. check this. but if all looks good and reseating it doesnt fix your problem. the pads are done.
oh and good luck finding a new set.
Natural selection has arrived. Only the strong will survive!