If you cannot visually see (or you cannot figure out) why it is happening, the rifle should go back to Ruger.
I presume there is something out of adjustment, perhaps just bent a little bit the wrong way! It sounds as if something is preventing you to lock the trigger assembly to the receiver due to something that has to do with the stock or with the metal inside the stock.
I remember, with my Mini-14 I used to own, I really had to press that trigger guard into place to make it latch-up (I remember holding the gun with both hands and using both hands to press the receiver and the bottom of the trigger guard together until there was a definate "SNAP")!