Hey, I have been trying off and on for the last year or so with no luck, my cycles can sometime be crazy. But when I was using opk's I didn't have a problem getting positives and when I was charting I ov'd within a day each month at the same time.. But I haven't charted since having a chemical pregnancy (except last month I restarted my temping) May 2010.
As far as the no positive OPK thing... Some women have a really short surge. so my advice is to stock up on the internet cheapies and test at 10am and 5pm everyday. Because it's possible your totally missing your surge if your testing once a day. And others (like me) I get a positive for two days in a row. Ovulation then occurs within 48hrs. Irregular cycles are usually caused by an ovulation issue.. So I would start using a bbt to take your temps every morning and try to get some sort of idea how your cycles are to see if theres a pattern. Do this for 3 months and if you feel there's a real issue and no clear pattern you have charts to take with you to see a dr.
When we were trying for my daughter we did the deed every other day and got pregnant first month.. Sperm can live for up to 5 days and it's best to have sex before ovulation and day of. Because the egg lives from 12-36 hrs.
I've done plenty of research on all this so if you have questions or want advice etc I'll buddy up.. My AF is due pretty close to yours this month.. Feel free to message me and I hope this has helped..