GI issues are very distressing and can be tricky to figure out. Another vet might not hurt just to get the ball rolling, but I might also consider an internal medicine specialist, if you have a specialty practice in your area, or a teaching hospital if you have a large University nearby with a veterinary school.
My own dog went through similar issues-- vomiting and diarrhea, sometimes vomiting up kibble 8+ hours after eating, plus weight loss, bad gas, etc. It was nearly a six month process to figure out what was wrong with him. I have a thread on the board about it that I'll link here:
At wit's end with chronic diarrhea
One thing you might try just to start off is switching your dog's food. Wellness is a great brand but there lots of ingredients, and even a single ingredient might be throwing off your dog's GI tract. You might consider switching her to a kibble that has a single novel protein source (a protein source she's never eaten before) and a single novel carb source. California Naturals, Nature's Variety Instinct, and Natural Balance all have limited ingredient foods, and Wellness has their simple solutions if you can find one that had a protein source she's never had before. Try her on the new food for several weeks and see if that makes any changes.
Other possibilities that wouldn't hurt to try:
Worms can be difficult to detect sometimes on fecal exams. Might not hurt to do a round or two of Pancur.
Same with giarida-- the giardi life cycle is cyclical so they won't always have the symptoms, so it's very difficult to pick up on a fecal exam. Many folks on the forum have had a lot of success with this product:
Good luck, and please keep us updated. GI issues are so frustrating.