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Check the primer on the next one that does it, if it does after cleaning. Even though it may eject, some bolts continue to move back as much as 1/2" after ejecting. Another possibility is wear on the hammer face, from the bolt riding on it, when camming it backward. If it were wore down enough, it might not catch the sear sometimes, if the bolt could not make it arc back enough. Though I would expect a lot of rounds through one for this to happen, but Mossberg has been known for using softer steel and castings in the past. A quick look at the hammer face, and the front body would tell.
The only other possibility would be a worn down firing pin, which might not indent a primer enough, if it was seated just a little lower than the rest. If not this, then check the hammer and gas system. However, you should be able to tell if the hammer followed the bolt forward when you cock it to throw out the misfired cartridge.
One last thing, clean out the gas port (hole) in the barrel. If it gets a little choked off, it can cause this too.
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