Well first 20% alarm would go off and then 10% and then 2%. When the fuel is less than 2% all alarms would go off at the same time and that could create like an extra urgency to change the fuel rod maybe?
You might be using more than 1 fuel rod if there are 4 fuel rods with 5% each for example, contraption will see that as 20% so check the guide in the description to do the alarm with multiple fuel rods. If it's not that you might be connecting the alarm's toggle_state socket instead of the set_state socket or you mixed greater component's signal_in_1 and signal_in_2 sockets.