sounds like the key fob is operating if the light is flashing, so it cant be a broken button. If the key fob is operating that must mean there would be either a problem with the signal or a problem with the receiver.
I would think the simplest reason maybe down to a low battery so maybe the signal strength is less so it no longer has the range it used to, maybe try sitting in the car and pointing it directly at the receiver or next to the receiver and press the button see if it activates or not.
it maybe worth just changing the battery to rule out that problem. After that I would consider the fob was working or at least sending a signal out but the receiver is either not excepting the signal or just failing to receive it. In either case it could mean a faulty receiver or somehow a loss of coding between the two although I always thought the loss of coding was more to do with the immobiliser not the central locking.
