Alright, then it's probably what opteron said. The valve may be installed backwards, or the o-rings are so dry that they are catching on the body (or both). It doesn't look like there is a regulator of any sort on there stopping anything else. Check for cracks in the o-rings as well when you have it apart. If you don't know how to disassemble it, it should be similar to this - http://www.spyder.tv/product-manuals...ed/imagine.pdf
. Just ignore all that gibberish around the trigger frame, you won't be needing any of that. It may look somewhat complicated, but it's really designed pretty simply.
If you want to take pictures of it disassembled, we may be able to help you more.
Oh, and I just noticed too. Your bolt is rear cocking and there is no guard stopping it from hitting you. That's fine, but just be wary that you don't stick your mask anywhere near it when you're shooting. It could easily crack the lens.