Blend fields must match EXACTLY. If you have dates in one source and date-times in the other, for example, you'll get no matches.
If you have date-times in one but not the other, then create a calc field in the one with times:
This will truncate the value just to the date. Then blend using that calc instead of the original date-time field.
Wow!!!!! Thanks Heaps. I've been working on this whole project for a while and got stuck on this last issue. You explanation and solution worked perfectly.