Even though most people probably would have gotten a BFP by now, I have heard plenty of people who don't get them at all or it's a while into their pregnancy before the hcg starts registering on a HPT. Maybe you are one of those few people?!?
My question for you is this, I know you chart, but were your doctors doing follicle following, bw, or anything to confirm ovulation? My SIL was on Clomid for several months and only actually ovulated a couple of those, and the only reason they knew is because they were doing the follicle following...
Finally, if I were you, I would wait maybe a day or two, but then I'd be calling my doctor. If he guaranteed you a 31 day cycle, then I would call him and ask him to find out why something hasn't happened yet!
g/l KUP