So ignore the fact that this link is associated with the "OV-watch" fertility monitor
OV-Watch Fertility Predictor - American Pregnancy Association
I think the stats it has are relatively accurate. You have a 5% chance of conception on ovulation day. So, no, it's not "impossible" (although as atomic said it's hard to know in your individual case if your gynecologist had information we don't have about exactly when you ovulated). 5% is just extremely low and if your attempts have typically been on ovulation day, I do think that's why you haven't gotten pregnant yet. Odds jump up to 26% chance of conception day before ovulation and 28% for 2 days before. If you use OPK and DTD at a positive OPK, you'll nearly always fall in that 26-28% range rather than 5%.