Scan/ trigger question

This will be my second cycle taking clomid days 3-7 and then trigger shot. Last cycle CD12 (when doctor wanted the scan was a Sunday). So I went CD10 and they said I had two good follicles BUT she couldn't tell if maybe I already released them or was getting ready to (is there a way they can tell!?) and they gave me the trigger shot, I didn't get pregnant. This cycle I took clomid on CD3-7, and again CD12 falls on a Sunday. But this time they want us to wait until CD13 for the scan and trigger shot. I guess my questions are 1) can they tell if the egg has been released or will be releasing? Like can they tell the difference??? 2) is there a negative factor to waiting until CD13?? Like can it be too late?