Forends become loose from previous attempts to replace them without first cocking the hammers. This will bend the Baker or 'J' spring, and straightening the spring slightly will usually tighten the forend to the barrels and lug.
 

 
Remove the forend iron from the wood, "tweek" the J spring a little, reassemble (with the hammers cocked) to check the fit, and repeat until it is tight.