i had the same problem up until today. my male is a bit shorter than my female, and every time he'd try to breed her he wouldn't be able to get it right, it took some trying and god bless his heart he's a trooper
and he finally got her today. they were tied for about 5 minutes or less. but he was so tired he was panting, i tried to take him away and just hold him for a bit to calm him down, but he wanted to keep on and he succeeded
how long have your pair been trying for?