This was common to a lot of these (they fire from an open-bolt, which is why importation was discontinued); the single biggest reason is probably going to be a lot of crud built up on either the sear or the sear-step on the bolt, stopping the sear from catching the bolt when it comes back. Take it apart and clean it thoroughly, including the bottom of the bolt with a toothbrush and blowing the trigger-group out with Gunscrubber and compressed air, and then lightly oil it and reassemble it. If it still goes FA, then you likely have a broken disconnector, and you'll have to start looking for parts.
Edit: I don't know if you've ever had it apart, but they take down pretty simply; unscrew the forearm, and the bolt and barrel should pull straight out the front of the receiver.