I bet she's not ovulating yet. The males just know most times. We read that about average cycle and start feeling that pressure for them to mate on days 9, 11 and 13: But sooo many dogs don't fit the same mold.
I do recommend seperating them and bringing them together a couple of times during the day to check, I bet in a day or so (could be more) he shows more than just "butt-sniffing interest" |