When I open Obs it's just a black screen. If I briefly switch to the Minecraft forge 1.12 window and back to Obs, Obs only picks up the pause menu. (I'm making a window and game recording neither works)
If that doesn't work (for whatever reason), take a new screen capture and crop the image with ALT and the left mouse button.
Attempt to make a screenshot NEW. Cut them to the window.
If it doesn't work please comment
Probably because you play it full screen. If you tab out then it only records the last movement.