There are different manufacturers of metformin, both brand name, which I believe is Glucophage, and generic. There are also standard and extended release versions. Specific medication nothwithstanding, some people find that they respond better to regular or sustained release, or prefer once daily or multiple daily doses. Then between brands of generics, some people (myself included) find some brands perform better than others. While they are technically supposed to be identical and have the same amounts of drug per pill, there is a small variance allowed (between 3 and 10% depending on your source) between the generic and brand name. There’s also an allowed variation in the pharmacokinetics and distribution in the body, called “area under the curve”, between the original and the generic, but again, this is supposed to be within a few percentage points.
One more thing to consider is that some folks try a med and say they don’t like it, but later we find out that it’s not the med, but the timing. For instance, some people find they aren’t benefiting from (or are suffering too many side effects from) a new med. Upon investigation, we find that the person has been taking it with other things like food or vitamins that prevent or overly enhance its absorption, or they’re taking it with other meds that actually prevent it being eliminated from the body and amounts of the drug climb above therapeutic levels.
tl;dr: There are lots of options, so if one isn’t quite working for you, don’t be afraid to ask your doc to try a different brand or release type.