Trouble moving hands to hold unique weapons

Hey guys, I have my model all set up for my mod. It is a grenade launcher attached to the standard assault rifle. The problem is, no matter what I try, I cant seem to get bones to rotate!

I did a “Retarget Anim Blueprints” from the standard rifle anim blueprint, connected it to the mesh, and the skeleton, so the model appears in the right spot, and spawns correctly. All I have left to do on it is reposition the left hand so it is gripping the launcher instead of gripping through the launcher, but I am thoroughly stuck.

Any help would be greatly appreciated. Thanks!

Edit: The first picture shows how I need it to look. For whatever reason, whenever I spawn it in, it defaults to the second picture.