backup in time machine does not go back far enough then yesterday why??

Likely, because Time Machine detected that your existing backup file was corrupted, so it erased all the old backups and started all over again with a new backup.

Usually....but not always....a message like the one that you see below appears when this occurs.

If you don't recall seeing the message and clicking to Start New Backup, then Time Machine did this for you.

Unfortunately, the message that you see above appears a lot more often than it should with recent operating systems like Sierra, High Sierra and Mojave. Not much that you can do about this other than setting up a USB drive that is connected directly to your Mac and backing up that way.