How long have you been on the metformin? Sometimes it takes a bit of time for your body to react to the medicine. I'd say if you've been on it for more than 2 - 3 months, and still haven't bled, then go back to your doctor and let them know.
I also have PCOS and like the other woman above feel that OPKs are somewhat wasteful for us, as they will almost always show up with a faint line for us regardless of what time of the month it is because of our levels of LH due to PCOS. What I found helpful last month was keeping an eye on my cervical mucous -- not tracking it, but just casually noticing. I DID track my BBT which conclusively confirmed that I had ovulated, and that in conjunction with the CM I was experiencing made me certain I had ovulated.
I would highly recommend charting your BBT, as I believe that is one of the only accurate ways to confirm ovulation for women with PCOS, and get a prediction of when it will occurr. Knowledge is power!
Hope that helps!