Thanks for all the replies.
I never thought it could be the nipple.
I do try and seat the cap firmly every time. After one or two misfires the cap does not have seemed to move any.
I will try to stretch the spring.
The #11 nipple I'm using is a replacement nipple. I bought it 3 years ago. I thought I took good care of it.
I do have a 209 primer breech plug and a musket cap nipple. They came with the gun when I bought it new. I just do not have the musket caps or the 209 primers.
I guess there are 3 options for me=
01. replace the #11 nipple
02. switch to the musket cap nipple and buy some new caps.
03. switch to the 209 breech plug and buy some 209 primers.

I'm leaning towards option 2. Either should fix my problem.

Anymore suggestions? What do most inline hunters shoot #11, musket caps, or 209 primers? Which misfires the least?