IMO if you can already run 300 then it's probably down to either luck or small execution errors.
Ever feel like your stroke needs work? Catch yourself steering or jabbing or lunging?
Does the run often end on a length-of-the-table shot?
Might need to work on your mechanics.
Is it more like you dog an easy one you should have made? Then that's probably just mental stamina.
Your brain can get tired even if your body is fine.
Other than that...
Books can help with patterns and decisionmaking... but I don't know what funky situation
could come up in rack 22, that didn't come up in the first 20.
I know some will disagree but I think your best bet for learning something new that will help you to the
the next level is to learn from someone who's already there. In other words you need to learn from
someone who's run about 400 balls. Guys who have run 250 probably can't get you there.
You should watch every DVD in this set -
https://www.seyberts.com/products/John_Schmidt_Mr._400_Straight_Pool_DVD_Box_Set-103702-230.html
Or else spring for a private lesson from John. Other than him, I believe Ray Martin is still teaching in FLA,
Thorsten Hohmann has run 400+, maybe allen hopkins or mike sigel? Danny Harriman?