I dont have alot of advice, but the theory of DTD when ever and it will happen was true for our first 2 kids. The 3rd we planned and timed.

I also found that when we were TTC #1 the more i stressed about trying to get preg...the more it didnt work. So when we relaxed and let nature take its course, it happened.

Perhaps stress of wanting to get pregnant or work commitments could be throwing your AF out a bit?

Not really sure to be honest. How long have your been TTC for?