Dog doesnt want to use doggie door

Question
I have a Chihuahua Mix (Fenny) that is about 11 months old and he is fixed. He wont use the doggie door and we are trying to get him to stop going potty in the house while were gone. We also  have a Dachshund (Shaq) that is about 8 years old, he is also fixed but knows how to use the doggie door. Both arent trained, we are trying to get them to become better behaved but this is our first issue to tackle.
When Fenny is following/chasing Shaq and he runs through the doggie door, Fenny stops imediatly as if he is scarred of it hitting or touching him. We have attempted to kneel on the other side of the door and call him, use treats, and his favorite squeeky toy but he just sits there and looks at us like were stupid. If we walk away he will just go around to the back door. And if we leave him outside he will just sit in the grass. Have any tricks?

Answer
Hi Payden.  You have 2 separate issues here.  You could remedy the dog door issue, but still have a housetraining problem.  Just because you teach him to go out the dog door doesn't mean he's going to "get" that he has to potty outside.  But he's not too old to learn, so here are some tips:

First, confine Fenny to a crate when you aren't there to monitor him.  As soon as he leaves his crate, he should be taken outside to eliminate.  Go out with him, be sure he eliminates and reward him.  Only then should he be allowed freedom in the house.

The dog door (DD) process I'm going to describe below should be done when Fenny is hungry. Skip a regular feeding time and a few hours later try this.  Smear a bit of peanut butter or soft, smelly cheese or a smidgen of canned dog food on the DD flap.  Confine Fenny to the room with the DD.  Be sure Shaq doesn't have access to the area.  Allow Fenny to investigate the smell on his own with no pressure.  Leave the room or sit far enough away to observe but do not interfere.  Be sure there's nothing else interesting to do in the room or make the space small enough so that he's tempted to go check out the smell.  You could put your crate (door open) up against the DD to limit his area.  Once he's successfully sniffed and perhaps licked the food off the flap a few times, you can move to the next step.  

Again, when Fenny has missed a meal and is hungry, take Shaq and go outside with some really yummy smelling food (liver treats, warmed up leftover chicken or beef).  Be sure Fenny sees and smells what you have before you go out.  Have Shaq on leash so that he doesn't go in the DD while Fenny is thinking about coming out.  Again, confine Fenny to the small area or crate around the DD. He should not have access to sit by the back door.  Have a training party with Shaq outside - teach him to sit and get a treat, teach him to walk nicely on leash next to you and get a treat.  Praise him and make a big deal about what you're doing with him.  Be sure that Fenny hears and can possibly see what's going on and that he is missing out on the action.  Other than that, ignore him.  Don't try and coax him out the door by lifting the flap yet.  Let him decide he wants to come out on his own.

Some key points to remember while doing this work:

1.  Fenny MUST be hungry to be motivated to try something he's very much afraid of.
2.  While Fenny is deciding if the DD is safe, take the pressure off him.  Don't force him out the door or try and physically help him.

The last alternative I have for you is to remove the flap from the door entirely for a while until Fenny is coming and going through the opening with no issues. Once you put the flap back on, you'll probably still have to do the steps I wrote about above, but at least he'll have an idea of how to come and go.

Please let me know if you have any questions or comments.

Good luck!