Silicone Keypad Design Problems
Electricity and water is the issue. The tin in the solder plating will grow a crystalline structure and form an oxide that doesn't conduct very well. Spent many months in the 1980s solving this problem and the bottom line is use gold plate. Don't be cheap on this. The company I worked for at the time sued the supplier a lot of money for their incompetence and they were big in the industry at that time.

If you can't seal it (and clearly you can't because you can clean the contacts) then water will get in. It's inevitable.