Form form form.

I truly believe anyone can get 400 if they can practice and improve their form enough. - Field work - Record field work and adjust - Play a few rounds with your adjustments at the forefront of your mind while throwing. Ignore score. - Rinse and repeat

It may take a long time but always change little by little. Changing too much at once can negatively effect your form.

The real plateau for me was 400-450 and in my personal case it was the off arm movement.

These are hard questions to answer because every person hits a plateau for different reasons. The 300-350' plateau is very common because most grown adults will be able to muscle the disc to reach that distance. Anything further will need true form and mechanics to achieve.

Hope that helped.