It may be that the remote button 'contacts' have worn out through use over the years.
Here is a link that shows how to clean the buttons on a TV remote control unit. The principle of operation for key fob buttons is the same, so perhaps you could check your remote key fob and see if cleaning the contacts solves your problem.
Alternatively if cleaning doesn't work, you could try re-programming your keys with the car. You will have to do all the keys for your car as re-programming only the one key renders the others useless.
Here is a link that shows how to re-program your keyless remote.
Pause the video before it ends and then click on the Show More link below the video screen to see the programming information. Take note of what happens to any other keyless remote(s) that you have for the vehicle, once you start to reprogram your remote. You have to reprogram them all.