I Take Metformin 500 Mg, Twice Daily. I've Noticed With

I take Metformin 500 mg, twice daily. I've noticed with my latest prescription refill, that I feel different than before like it's not as effective as before. The one I have now is imprinted with Ip 178, but I think the ones I took before had a different imprint. Is there a chance I was given the wrong "type" or maybe there are different ingredients in this one I'm taking now? I will check with my pharmacist but just wanted to know if different manufacturers include different ingredients than another . . . . Thanks.