The elbow may be restricting plus I think your stack may be too long to draw, best bet would be to check out the pit calculator and see if your numbers are correct.
If you end up replacing the stack, I would shoot for a a 4" stack or a 5" stack, straight out of the top or side with no bends.
You could try taking a foot off of the stack and doing a test burn to see if it brings the heat up.
IIRC for an 80 gallon tank and a 4' stack its 25- 26" high, I just did a test burn on mine.
I loaded up the firebox with 5 logs and fired up the pipe burner for 5 minutes, after about an hour I had it up to 450 degrees.
Do you have the wood raised during a burn?
Does your stack go to grate level?