I would suspect that it may very well have to do with the fact that they are a bit too young to be bred at this point. You should wait until they are both older...especially in the case of your female. Breeding before she is fully matured is only asking for some serious issues. In addition to not being fully matured adults, they are not yet at an age where you can even determine if they SHOULD be bred.
My advice would be to wait until the next heat, examine if they represent good breeding stock, and then try again.
And welcome to YT!