Hi there,
I think it is very common for Zantac to only work for a short time, either because babies can get immune to it quickly or because they outgrow the dose quickly. With my first son, we got 2 or 3 good days out of it. With my second son, we got ONE good day. Then we moved onto Prevacid and things have been better. We do, however, still use Zantac in the middle of the day (Prevacid morning and bedtime).
I would definitely ask the doc about Prevacid or another PPI as soon as you can.
Good luck!