Take a couple of shots of the inside of the receiver.
First guess? Take out the stock/grip bolt and add a couple of washers to it before reinstalling it. Second, have you changed your safety by any chance?
The common mistake that usually causes that problem is folks installing the pistol grip using a longer full stock bolt. If the bolt extends into the receiver so it crosses the safety connector path, it breaks the safety connector tab off the trigger and can break the trigger itself. That stock/grip bolt shouldn't extend out of the hole inside the receiver more than a hair at most. If it does, washer it inside the grip/stock to shorten it.
If you have a Mav 88, you can take the trigger out of the Mav and replace it back into the 500. The Mav trigger is identical and that extension part is only used for the 500 safety function. It's not necessary for the Mav 88.