A slip mating is possible, but unlikely. It sounds like this would be your female's first breeding? It is quite uncomfortable for the female, if she is yelping and jumping then he wont be able to get the tie. If you really want them to mate the best way is for you to hold her still until they have tied and then keep both of them calm (yes, she is going to yelp! you will have to hold her, she will yelp right before the actual tie and during the tie and that can last up to 40 min. just keep her still and try to comfort her, DO NOT TRY TO GET THEM APART even if she is freaking out! just calm her, if you pull them apart you can cause serious damage to both, ice, water, etc. will not help.. they have to wait it out). What is happening right now is that she is yelping (and I assume jumping) right as the male starts to go in, which means that he isn't actually getting far enough to really get a pregnancy. Be happy that you have little dogs though and assisting them is A LOT easier than trying to assist when you have to deal with a 90 lb male. Even after they mate they will continue to try so you are going to have to control when they are together to mate, you should not just let them run together to mate at all times, put them together once everyother day (best for your female, otherwise she will have to keep going through the pain), dogs sperm lives for 2-3 days so if you are getting a tie every other day there will be enough in there at all times to get her preg. Good Luck!