If the bolt resets, the the problem is probably not because the sear is too far back. If the sear was too far back, the trigger would be heavy, but the bolt would not reset.
Put the valve into the body with the bolt but without the bolt spring on it. Don't air it up. If the bolt is back against the bumper in its normally reset position, you should be able to freely rotate the sear to engage and disengage without any interference. If the sear hits anything in this position, then you have a problem. Its easy to diagnose without air, because there is nothing to add force so all friction and force is caused by the sear action.
Except for the Automag in front, its usually the man behind the equipment that counts.