Mating Chihuahuas

Question
I am currently trying to mate my four and a half year old female chihuahua for the first time. The stud I've chosen has fathered one previous litter about five months ago with another bitch. Her first sign of discharge was July first so this is about her twelfth day. He mounted her for the first time yesterday and they've been going at it all day today, unfortunately they have yet to tie. She's been showing she's ready, curling her tail to the side, backing into him, but everytime he mounts her he seems to have a lot of trouble getting his penis inside her vagina. I've held her for him but whenever I try to help him out he stops. Is there anything to be done to help them along?

Answer
Praise him and let him know that it's a good thing -- what he's doing.  The alternative is to have someone else help him and you leave the room.  If you are the "person in charge" he may be nervous with you around to watch.

The alternative is collecting and doing an A.I.

Good luck.