When I was just starting out, one of the mistakes I would make frequently is that I would presume too much of my players. The grids for a possession exercise would be too small, the amount of defensive pressure during attacking exercises would be too high, etc. Especially for the youngest players, I found the speed of play I thought they should be capable of, and the speed they actually were capable of were usually at least a level or two apart.
As I've gained experience, that's one area I've consistently improved in. Today was the first training session for my B07 team. We added some new players, and are moving from 7v7 to 9v9. We worked on building out of the back under pressure today, 5v2. For a so-so CSL Bronze team, 5v2 is a sufficient amount of pressure for them to overcome at this time. This team has some experience circulating the ball, but are far from mastering it. Additionally, the field we played on was horrible.
Especially when learning or refining a new aspect of the game, I've found that around a 50% success rate is the sweet spot. It should be easy enough that they know what it looks like when done correctly. For example if my back 5 turned the ball over every play, and never got close to building out, I would have no issue reducing the pressure to 5v1. As they learn and improve, you'll need to continue to ratchet up the pressure, reduce the space, and find other ways to push the players to improve.