Yep, it can take 6 months to see if the met really is working, and you also may need to up your dosage over time. Many docs prefer regular met to the XR because that is what most of the research has been done on...the regular. Although, I've heard of some people asking for the XR if regular hasn't worked for them (ditto for requesting glucophage vs. metformin...some people swear one works better for them than the other).
I was like you...I low carbed for a while when ttc, then went on metformin 1500 mg, built up my dose for about 3 weeks. We added clomid (I had already done several other clomid cycles that never made me O) and I still didn't O. I was given the option to wait out the met/clomid or move onto injectables, and I opted for the latter as I hadn't O'd in 8 months of ttc at that point. I was ready for gonal F. I did eventually conceive on the gonal F (2nd cycle, but it was the first ovulatory one since I overstimmed and was cancelled my 1st cycle). I'm curious if met wouid have worked if I stayed on it w/ clomid longer, but I never regretted moving onto injectables. I stayed on met through my first trimester.
It may take some time. You may need to have your doc play w/ the dose. Hang in there, and I hope it works for you
beth