VLOOKUP returns values for some Lookup values but not others
255 Character limit on VLOOKUP
VLookup has a character limit, as soon as you go over 255 characters, it won't work for what you are trying to do.
If your lookup value has more than 255 characters, you will have to find another solution such as using the LEFT function, RIGHT, manually doing it, etc.
This was my problem.
Edit:
It seems this is another solution and the character limit isn't a problem: XLOOKUP