From the FG you have your opponents head & arm trapped within your legs. You opponent is defending making it hard for you lock in the submission.
The Reverse Triangle demonstrated in the video by Marcelo Garcia is a great variation for when your opponent defends by locking their hands under you.
What you are going to do is change the angle by going onto your hip so that you are able to close the Triangle. Then instead of pulling your opponents head down to get the tap you grab your shin and pull it toward you.
I really like this option. It is one that you will have to work on in order to get the angles right for the submission but a great finishing move for sure.
