I purchase SAF Instant Premium Yeast from Amazon over a year ago and used 6 grams in my whole wheat recipe and the loaves never fell. When I ran out I purchased the identical yeast and started have fallen loaves. I kept reducing yeast and at 4 grams I got a fully risen loaf. Next loaf fell and after that increased my salt from 5 1/2 grams to 7 grams. No help. I then went to 3 1/2 grams of yeast and got a full loaf. I am now thinking my first purchase of yeast was defective. Dry yeast will last for years if kept under ideal conditions but I suspect my first purchase had been old/overheated or possibly a bad run. This is the only thing I could think changed. I measure all ingredients and am careful with ingredient temperatures so it had to be the yeast. I just had a hard time believing that I had to almost half the amount of yeast to get the proper results.
I am inclined to believe the yeast may have been old or damaged. This is because I have purchased some things from Amazon that have been from resellers that may have stored things improperly. I believe I will stop buying yeast from Amazon but from a more reliable source.