Hello, Sophie.
A light line should still be a positive reading. It could also mean that you're about to O or have just finished O'ing. If you make it a point to BD on days 10, 11, and 12, you should be good. Plus you get the added benefit of lots and lots of ... well, you get the idea.

Another thing is that stress will inhibit O-tion, along with many bodily functions. The best thing to do is trash the OPK's and go to town in the middle of a cycle.