Missdee, I have scans every month
It basically a scan to look at your ovaries, to determine whether there is a follicle growing or not (which will release the egg) - the follicle grows as a result of the drug (clomid). A scan can determine when you will ovulate, depending on the size of the follicle - generally, they're looking for anything over 18mm (or around that: different places will have different guidelines).
My scan usually takes place on day 12 of my cycle, because my and my fs have realised that that's when I've responded to the drugs fully, and will be about to ovulate. It means that we can pinpoint with a fair amoung of precision when is best to BD
as for the responding to oe cycle and then not the next, despite being on the same dose, I find that I don't respond when I have had a "natural" bleed from taking the clomid/femara the cycle before. When I take provera to kickstart a cycle, I usually fully respond to that cycle. Maybe you're the same?
it's definitely worth noting whether you're similar, because it means that you can take provera before every cycle and ensure that you will respond the best you can!
Hope this is helpful. sorry it's so long!