A flush is filling your pot with 3x its size in water. (if you use tap test ph first otherwise use distilled.)
I only flush twice during the whole process. When swiching from 20/4 to 12/12 so I can start fresh with
flowering nutes(sometimes I don't even do a flush then), and another flush 2wks before harvest.
You can also do a flush if you over-feed.
most of your problems are over feeding, too many nutes. FLushing helps clean the soil and you can start again depending on the age of your plant. When your leaves start to turn a yellow color, flush the plant.
If your leaves turn yellow it could be a million things, make sure the yellowing looks like nute burn,
( go to the plant problem section) before you flush, cause if they are turning yellow cause of a deficiency flushing might make it worse.