Black Lab Puppy diet

Question
Hi - I have a 9 week old black lab puppy and am having issues finding the right food for her. We got her when she was 6 weeks (young I know, but it was a rescue case) and she was eating Purina Puppy Chow. I was told by a friend to switch her to a better quality food - so we gradually switched her to Purina Pro Plan. During the switch she began having loose stools - often going 6-10 times a day! Once they became unformed, I called the vet. They did a fecal test (negative, but she has been trated for worms) and put her on a medication to level her digestive tract and I was instructed to feed her Percription I/D (the magical canned food) for 4 days. Did that - then began introducing the Purina Pro back into the diet mixed with Purina Puppy show (3/4-1 cup total/meal) mixed with 1/4 cup hamburger/rice mix (as instructed by the vet). I started out 90 Puppy/10 Pro, then changed the percentage by 10 each day. Everything was fine until the kibble mix became 50/50, then we went back to the loose stools again. She is back to going 6-10 times (even at night, which she was sleeping 8-9 hours). Do you have any suggestions? I am going to call the vet tomorrow, but I dont want to keep going in circles with this issue. I know dogs have a hard time digesting corn, so I did pick up a food without corm that I might start switching to (Science Diet, Natures Best, Puppy chicken and brown rice). HELP!!!!
Thanks ~

Answer
I know of no other subject than dog chow, where so many people have such strong opinions with no facts to base them on.  Go back to the Purina if it is working.  Even many of the lunatic fringe will concede it is a good idea to stick what is working.  Most dogs will do fine on most chows.  I am surprised yours seems to be having trouble on the Pro Plan, but if it is, stick to the Purina.  Much of the negative info on corn based chows originates with those selling more expensive chows.  It is then taken up by those firmly convinced if it cost more, it must be better.  Don't let them stop you from sticking to what is working.  

I have had good luck with both Purina and Pro Plan.