Car not starting issue possibly the immobilizer

I recently replaced the car battery in my Toyota Aygo. I used the main key fob with buttons to drive the car for a couple if days following replacement of the cars battery. Everything worked fine. Yesterday I tried to start the car but it would not start. Plenty of power but would not fire up. I tried to start the car with my single key and it starts every time. If I start the car with the single key I can then use the main fob key to start it again. But only once. If I switch off and try and start the car again with the fob key it turns over but will not start. If I start the car again with the single key I can then use the fob key again but only once. Confused!