Emily was on prevacid or some form of it for quite a while. The first year seemed like one big experiment to figure out how much enzyme she needed, which antacid worked best, how and when to give teh antacid...and as soon as we'd get it figured out, it would all change a week or two later. After she turned one and started walking, things really fell into place for us, at least as far as enzymes/antacids go. She hasn't had an antacid since then, and I don't think we've changed brands or dosage of enzyme since she was about 18 months.
I hope you can quickly find what works for your little cutie!