The "New Row" trigger sees the most recent 50 items which affects how the maximum amount of rows your Zap can trigger on at one time. See deduplication for more information.
A common problem with database services on Zapier is deduplication. You can read more about deduplicate in general, here.
This applies for all the database services on Zapier like MySQL, MSSQL, Postgres, etc.
Zapier deduplicates off of the primary keys on the table you are triggering new items from. If you have multiple primary keys then a concatenation of all of them is used to deduplicate.
The ramification is if you make a sweeping change to the primary keys on your table, you'll inadvertently Trigger tons of things through your Zap. Sometimes this is caught by Zapier's flood protection feature. Nonetheless, you should turn your off Zap before you make changes and turn it On again after the changes have been made. This allows Zapier to establish a new "baseline".
If your table has no primary keys, Zapier will guess which column to use, usually looking for column names with id in it.
I Don't Have A Unique "ID" Column
If you don't have an id by default, you must create one or return one manually with the query, for example:
SELECT *, CONCAT(user_id, created_at) AS id
FROM my_table ORDER BY created_at DESCOr whatever technique most closely approximates how you want Zapier to consider your rows unique.