If the shaver is old, it may be time to replace it. All rechargeable batteries slowly lose their ability to hold a charge after a period of time and eventually become useless. It's impossible to know how long that will take, but if it's a few years old, chances are that's the issue.

If it's not very old, it could possibly be a problem with the charger itself. The only way to check that is to try another charger. It may be worth taking back to wherever you bought it.