Leelee it could be a combo of things. You may not have ov yet- what day of your cycle are you up to? I don't ov till Day 17-19 although day 21 sin't unheard of for me either lol. Also Sometimes you only have a short LH surge so testing twice a day if you can might mean you don't miss it. I would normally only do that though if I thought my CM was begining to look a bit fertile IYKWIM. 
Its possible that you're not ov this cycle, I'm not sure why that would be but I think that's prety common too? 
With your CM- are you just checking on the loo paper or are you actually having a bit of a sweep up 'there'? (sorry if TMI). Sometimes I think some women don't produce loads of CM and I for one some cycles have to check a little more um indepth to find it lol. Sorry TMI but hey we're all in the same boat.
There are lots of women on these boards waaay more experienced than I am with OPKs but they certainly seem to work really well and really clearly for me so hope they do for you too. Good luck 