It is quite common for people who have them to arrive in France and find they don't work because the battery is dead. If it doesn't beep when you get to the péage, the battery is low and you need to change the transponder. These are available at any of the offices next to the main exits. If it is dead and you can't get through the barrier, go to one of the payment booths and she will scan the bar code and let you through. However, most of the exits are now unmanned and automatic.
Maybe this will help. It shows where their offices are on the autoroutes towards Le Mans:-
http://www.cofiroute.fr/cofiroute.nsf/web/autorout...
On the Sanef site go here:-
http://www.sanef.com/fr/institutionnel/groupe/prof...
then click on "la carte de réseau" where it shows "zoom". You'll see little red dots where their offices are too.
Any of these will change your transponder for free.
