Right so I don't know for certain if this is the problem but see the screws you put into the Trigger Box to attach it to your Upper Receiver they have Spacers and if these break and the Screws are too tight due to this missing spacer the sears dont connect and the rifle wont bolt like you said i had to change to a metal trigger set due to the stock one being plastic and the spacer breaking on it on my Well MB01 i have attached a picture of where the spacer is and circled it in red for you to see
http://postimg.org/image/6mt0v8s07/
Best of luck
Fenris 360
