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Loss of Power

Last night I shot a rabbit and quickly pulled back the bolt for a second shot but the bolt would not engage forward and was stuck back. I pushed and pulled about 10 times and the bolt pushed forward. Luckily the rabbit did not need a second shot and I put my rifle away. After work today, I examined the rifle and took some test shots. It turns out the Daystate Huntsman XL is shooting 20mrad low at 37m, where it has been zeroed and sounds very weak, I can actually see the pellets fly towards the trap. Just the other day I took a squirrel at 67m without an issue. I'm hopping for suggestions on what the issue might be and if I should attempt to disassemble the rifle myself?
 
Thank you both for you answers. I did place a piece of tissue over the breach and it did indeed move when firing the rifle. I took the o-ring off and it did not appear to be damaged so I placed it back on the bolt, but reversed the face when doing so and low and behold, the tissue moved much less when firing. I seem to have regained most of my power as well. I will be ordering replacement o-rings today.

Here is a picture of the o-ring for anyone interested. It seems to be in quite good shape but is slightly out of round.
 
I like the idea of putting oil on the o-ring but the Huntsman manual seems to be very clear about not putting lubrication on anything where it might get in the barrel.
Thoughts?
LUBRICATION: The bolt shaft should be lubricated with a small amount of light grease. Do not allow any grease to get onto the pellet channel where it could enter the barrel. DO NOT OIL OR GREASE any of the internal parts such as the striker or hammer, the inlet valve system or the snap-on connector (filler valve). Doing this could cause problems with consistency & unit sealing.
 
Silicone lube put a small dab on your finger and rub it into the oring so it is ahiny. Then install it. As orings age they get hard and don't seal as well also the NBR orings wear as they move. Polyurethane is a good oring material for probe applications. Every tin or so I apply a little more. Had 20k rounds on a breach oring and pulled it to see I how it looked. Was fine.