Accessing older adjustment layers in Photoshop History

You have just so many history stated for a document.  They are used round robin,  You effectively knock off the oldest history state when you do the your  next step once you used all the history state you have set in your Preferences for  your editing  history.  History states are expensive they consume a lot of machine resources If you keep a large numbers and have many document open you may start to run out of machine resources and Photoshop operation will go to pot. In your Photoshop's Preferences Performance section you can set up the 1000 history stated  to keep for open documents.  I would not recommend setting anywhere near that 1000  I think Adobe default setting is small like a dozen or so.  If you use complex scripts that perform many steps. I would suggest you modify the scripts to suspend recording history  so you can backed out what the Scripts did  by just stepping back one history state and the script will not purge out all the steps in your documents  history that you performed before using the script.