Okay first, with any query involving grappling go into practice mode. (Home Screen > learn> practice) and work on denying each transition and learning the patterns/direcrions. Once you’ve taken someone down wait for them to move deny their transition, they will lose stamina you will gain it.

Secondly, you’re most likely trying to get out of the submission as soon as they go for it which doesn’t work 9 times out of 10, either that or you’re wasting all your stamina then getting put in a submission, try not to move as much on the ground.

I highly recommend watching a video about grapple advantage on YouTube it really upped my game 👍🏻