Documents not on Time machine backup

Hi I have used Time Machine to transfer my documents to a new Mac Book. Unfortunately I am discovering that not all my documents were included. I still have my previous Mac Book-- this new M3 is my fifth or sixth-- and have retrieved a document that was not found on the Time Machine Backup. My question is not just how is this possible? (I almost cleared out the hard drive of the old Mac Book just last week) I thought that Time Machine was a trustworthy method to transfer data to a new machine. A more pragmatic question is how do I proceed to find and systematically transfer all the documents that have not been transferred to my new Mac Book. Thanks.