Build plate adhesion, PLA
From here:  https://io3dprint.com/review-anycubic-i3-mega-ultrabase/


  Ultrabase Bed
  The Anycubic i3 Mega Ultrabase is the latest version in the Anycubic i3 family. As hinted in the name, the main upgrade from the previous version is the Ultrabase bed. This is a textured coating on the Borosilicate glass bed that means you don’t need to apply any glue or tape to the bed to make your prints stick to it.
  
  Ultrabase is similar to the popular BuildTak beds except unlike BuildTak it doesn’t wear off and the most significant benefit is that parts are exceptionally easy to remove once the bed has cooled.
  
  The Ultrabase surface has a Moh’s hardness of over 7. This means you can safely use metal scrapers and blades to clean it without risk of it scratching!


Perhaps it was just not cleaned sufficiently from a prior print.