As Karen has said, first step is to rule out medical issues. A vet check is in order.

After that, I urge you to take Margie to obedience classes. Everyone in the household should attend; most places charge "per dog," so having extra folks present is not a cost issue.

Look for a training place which teaches positive reinforcement training. You may be able to find one using this link:
http://www.apdt.com/petowners/ts/default.aspx

APDT - American Pet Dog Trainers.

I do not suggest folks use PetsMart or PetCo or similar places. Often, the trainers there have passed a written test, but have never owned a dog. You want an experienced trainer, who works with his / her own dogs, in Agility, Flyball, RallyO!, Field work or herding. These are folks who are daily reading dog body language. They can teach you how to read your dog.

I also encourage group classes, not the more expensive one on one classes. You learn so much from observing other owners and dogs.

Keep in mind, a dog doesn't learn much in a one hour once a week class. Home work and practice exercises are a MUST DO.

Let us know what you find out at the vet!