
This is quite a common thing in maiden btches. Has she been checked for a stricture. If all is clear (you can check with a lubricated clean finger that there is no obstruction) then holoding the butch under the tummy and by the collar at the critical mownt, and quietly reassuring her will help, especially as the dog is also inexperienced.
It could also mean she is not quite ready. I have been in this situation, amd ended up with slip matings, so am trying again this coming season.
Sometimes getting dogs mated is not as easy as it may seem.