The decay theory- when someone has short term memory and stimulus is too weak to hold onto information
displacement theory- when you also have short term memory but this is because too much information is received at a fast pace and you can't hold onto it all.
inference theory- when you partially remember but not fully.
retrieval failure theory- you know the information but it can't be sought.
I find the retrieval failure theory most compelling because you know the information 100% it just cannot be accessed. I find that weird that your brain doesn't have the ability to do so.