I have had this happen way back. It was more than likely a low battery, but it is also possible that the bolt was not properly lubed. I did both at the same time after troubleshooting for a day, so I cannot be sure which one it is, but I have seen many people with low batteries having the click thing going on.
My battery was pretty new though, and I had only put about 200 shots through it when it started. However, if you are still using the Proto battery that came with the gun, throw it away. Those batteries are grade A crap. Grab a paintball 9volt, or a good lithium.
this happened to me. the front of the bolt came slightly unscrewed from the back and kept getting caught on the sear; you fire a few shots, ramp a little bit, and then boom, nothing but clicks. i had to get the front part loctited back on to the back (apparently it's not supposed to unscrew.) check your bolt and see if this is the case.