Some chunks not generating at all

Basically, three of my chunks have not generated, I don't know why as the tops of the chnks have generated just nothing beyond the top maybe 30 layers.

This is my survival world, I did back it up, and wanted to check if everything was right before I stated playing, and found 3 missing chunks on the border of my world, I haven't checked all the borders, as my world is like 5000*5000 and is a 5-6 year old world.