Why aren't you getting pregnant?

Louise
I'm 28. 
Been TTC concieve since 26. 
Out of curiosity what is the main cause in your 20s in your experience as to why your not conceiving? 
I've had all my bloods done, normal. 
Byf sperm normal. 
USS normal. 
I suffer painful periods but only for 1 or 2 days. 
I've never conceived...