What are you actually doing? (apart from the obvious )
If you're charting and observing mucus and all those things then I shall bow out now but if you're just doing the business then I have found both times (pg with no 2 now) that when I thought 'oh for goodness sake, it's never gonnna happen' and started making a point of making sure it was every other day right through my cycle (which was 5-6 weeks). Also when I got to that point I started putting my legs up against the wall afterwards. Don't know if the gravity thing is meant to be an old wives tale and I guess the whole thing could just be coincidence, but that seems to be what worked for me.
I if were you I would do it every other day all the time for as long as you can cope with. This is what I'm meant to be doing but I'm telling you now my dh is just not getting any tonight, I'm fed up! Good luck. The timing thing is so tricky and am in similar boat. If people say to me 'best thing is to just enjoy it and do it as often as poss' I want to scream....But, think it's true. It's how 2 dc's were conceived anyway!
When I started TTC, I didn't know anything about all the cervical mucous, cervical position monitoring - OPK's and luteal phases were a complete mystery to me. I also thought I had a bog standard 28 day cycle.
However, I now know I have an average cycle of 32 days, varying between 27 and 41. Ovulation therefore takes place for me anywhere between CD 13 and CD28!
I don't think that's the same for the average person, but from experience I think the choices to ensure pregnancy are either to start shagging every other day starting from the day AF leaves until she arrives again, which I found quite hard work , alternatively you can pay fairly close attention to determine when you ovulate, which worked for me after a couple of months...
Try the wagonners thread on the TTC board for support and advice, they've got a pretty good success rate!