Just tripped over this thread since I was having the same problem. Lots of good tips here, but in this case I think dpkrause was onto the proper cause: the tobacco was too dry. Here's why I think his first hunch was right:
I had been enjoying multiple bowls of Lane 1Q between 2 briars. I'm a slow smoker and I always clean my pipes after smoking. I stored the 1Q in a canning jar. Bowl after bowl was just great.
The 1Q sat for a couple of months before I came back to it. I let a few pinches sit for just a few minutes as usual and it smoked great. Just a few days later, though that last bowl smoked well, I remembered how prior smokes a couple months back demanded lots of relights, so I decided to let it air longer than usual. That bowl was very ashy and had none of the 1Q taste. I thought it was the pipe so my next bowl of 1Q I also let sit out longer than usual, smoked it in a different pipe, but got exactly the same ashy taste. Virtually unsmokable--I really didn't want to finish the bowl.
The ONLY difference was the drying time: same batch of 2 month old 1Q; short drying time = great smoke; long drying time = ashy taste in 2 different pipes. So I know that is the cause. Different blends may vary and there may be other causes in some instances, but this is definitely one cause for some blends to taste ashy.
For my next bowl, I'm going to let it air my usual time or less and I'm betting it'll be just fine, just like the bowl a few days ago.
Hope this helps anybody else with the same problem. Oh, I guess if you find yourself stuck with something too dry in your storage container, you could probably use one of those moisture discs to rescue it.