It is likely, that the threads are stripped in the block. Remove the stripped bolt and examine, the stripped threads wiil be attached to the blots as it is drawn out.

You will need to get either a helicoil fitted or a time sert fitted, I prefer the latter. It might be an idea to have all bolt threads to be time serted, I experienced before after one stripped thread, refitting the head another thread would strip at the final torqueing stage.