This is a know issue with some Knight bolt type rifles. In most cases it is a very simple fix.
The fact is that the hammer is not reaching the cap. There can be a few reasons why.
1. The most common reason why is the secondary safety on the back of the bolt is not screwed all the way back and tightened in place (it can vibrate forward if not snug).
2. Assembly of the bolt may not be correct; especially if you may have disassembled your bolt, removing the hammer assembly from the bolt housing and then re-installed it. If it is not assembled correctly the hammer may not extend far enough,
3. Your current breech plug may be to short - either because the Plug it self is to short or you have used an after-market and shorter nipple in the breech plug.
After checking the items above - open and close your bolt, hold the trigger back when closing this will allow the hammer to move forward. Visually check and see if the hammer is touching the nipple or
very close to touching. Next check the secondary safety and verify that it is not resting on the hammer cocking piece. There should be a small gap between the bottom of the secondary safety and the cocking piece. You should see one thread of the hammer assembly in the gap and NO red color of the stripe that goes around the hammer assembly that indicates the hammer is cocked.
To disassemble/assembly the bolt correctly screw the secondary safety in as far as it will go - make sure it is all the way in. Looking from the rear - now turn the cocking piece clockwise until it stops turning. Then turn the cocking piece back counter clockwise on full turn and align surfaces. Hold the cocking piece in place and turn the secondary safety all the way out.
This picture is an attempt to help with the alignment. When assembling the bolt surface A and surface B need to align. As well the striker tip and detent in the bolt housing.

Now if you feel everything is correct and the hammer does not reach or get very close to the nipple stack the nipple - you are probably going to need to contact Knight for an updated breech plug.
Sure hope some of this helps