Ovulation... Help please!?

I have been TTC for the last 8 months, with no luck. Used Glow the whol way through. We have done the deed every 2-3 days throughout, more often through predicted fertile window but still nothing! So the last 2 months I decided to use opk's, first no the I used the sticks but couldn't tell which line was darker etc (if any) so this month we have tried Clearblue. I am on Day 15 and still no surge so was wondering, how late can someone ovulate before you give up testing? I'm beginning to question whether I have ovulated at all the whole time. I have monitored CM and that has led me to believe I have but now I just don't know. Thanks all xx