yes, it does become less effective after a few weeks, I was told this by the pharmacist and my paed. I always found my DD did better at the start of a new bottle.
My DD is still on losec for her reflux but she has the tablet now, which I dissolve and put on her breakfast, it works much better and is even more effective as my paed said its more potent and "fresh". Maybe you could talk to your paed bout this at your next appoint.