@papayalily‘s answer above gets a GA. While it’s possible to have developed tolerance (i.e., requiring increased dose to get the same effect) this does not usually occur with H2-histamine blockers or proton pump inhibitors, which are the two most common kinds of reflux medications. It’s more likely your diet, eating habits, or your underlying condition has changed. See your doctor.
Tolerance, by the way, usually involves increased drug metabolism in the liver or, less commonly, changes in cellular receptor sensitivity. It has nothing to do with the immune system.
The differences among different manufacturers of the same drug (generic vs. proprietary, e.g.) is overrated as a factor & is usually important only with dosage-critical meds such as digoxin or warfarin. Generally store brands & other generics are reliable.