Welcome to HCM.
Good follicle size is around 22 to 25 mm. and after it ruptures and ovum release which occus on 14th to 17th day of regular cycle.Complete successful intercourse is necessory on that day to conceive.In your case on 17th day size is 15mm is too small and no chances of release of ovum.
Go for hormonal
study and start treatment accordingly under guidance of gynecologist.