Probably a silly question...

[TheDuchessofBoxford]

I am currently on cycle 3 of ttc my second. I'm not temping or using opk but am using an app to track dates and usually my CM is a good indicator and ties in with app days.

So if I am sure as I can be about ovulation happening and make sure we are DTD every other day throughout (and for a few days before and after to be sure) what are the reasons it might not happen?

Obviously there is more to it than sperm meets egg but what are the other factors?

Taking your temperature will show if you have a temperature shift and have therefore ovulated - your body can prepare for ovulation and you can still get the CM, but until you've seen a temp shift you can't be sure you've ovulated each month. I've used this method for not falling pregnant as well as now TTC - I have a book by Dr Jane Knight which is very helpful. Also read this link - trust this helps?
www.fertilityfriend.com/courses/lesson.php?p=1;7;0;0

I would DTD less, and be more targeted. Are you taking your temperature? That was spot on for us and I conceived 2nd month of trying, after nearly 2 years of being unsuccessful.

GP said DH should store up sperm by DTD less (includes masturbating, obvs), so that when we did DTD after ovulation he was giving it all he's got IYSWIM.

I think most of the time there is no specific reason. Everything can be perfect but for some reason the sperm just doesn't make it to the egg in time or at all. Or it does but it doesn't manage to implant. I think the statistic is something like you have a 20% chance of getting pregnant each month.
All kinds of things can make it harder like poor sperm or egg quality for example. Not every egg we release is a good one.