
In my breed many males , especially ones that have already been used or are older refuse to mate a bitch until she is spot on, and often this means they will only mate her on one to three days (the actual most fertile ones), it's often the young/inexperienced males that will mate any bitch that will stand still long enough, so be thankful that you have such a boy who knows his timings, and the bitch who missed may have been too early or even, as he was half hearted, may not have ovulated at all.
I had this with one of mine on her last litter, the male was just not very keen, mated her half heartedly, and she missed, the next season, even though she had a split season, he was much keener and we had a lovely litter.
Her daughter had been mated to him two years earlier, but as he was showing no interest up to day 16, she was progesterone blood tested, and this showed that on day 17 she had not yet ovulated, but was close, he happily mated her on day 18 - 20, so I knew he was to be trusted.