[RWT]

Put back together wrong? Wrong parts?
If it worked before you took it apart and does not after reassembly ?

 

[hmb]

Take the new forend support out and put the gun back together and see if it locks open. HMB

 

[Animal]

Locking lug? Do you mean the locking block? If so, scroll down to the parts listing in the below link and identify the part you are referring to, it may help to trouble shoot your problem. Also, double check the link and make sure that it is in the correct orientation and sliding smoothly in the tracks. Good luck

http://stevespages.com/pdf/remington_1100-1187.pdf

 

[Reuben Sandwich]

HAHA. Thank you. After a couple days working long hours in the cold, that made my night.

 

[dickgtax]

The disconnector on the trigger isn't engaging the interceptor latch on the inside of the receiver.
The disconnector is that little tab that extends from the side of the trigger. The interceptor latch is that little spring loaded lever that pivots on the inside of the receiver. Make sure it is rotating freely. Bend the disconnector out a little to make sure it is contacting the interceptor latch.

That should fix it.