My vlookup is not working

I've run many vlookups perfectly fine without problems however I have come across one that I cannot do.  I cant post the 2 spreadsheets because of the sensitive data on there but essentially when i write the formula in a particular cell where I was the vlookup to be conducted, the "=vlookup blah blah" appears in the cell in question.

What could I possibly be doing wrong?  The spreadsheet I am trying to lookup has the lookup client name in the far left column and there is no left or blank column on the spreadsheet.

Also please confirm when or where or what the rules that apply when doing a vlookup in the following situations:

- where the spreadsheet that is to be looked-up does not have the identifier in the far left (first column)

- where the identifier column in the looked-up spreadsheet is not in alphabetical order

- where in the workings speadsheets (that contains the formula) the identifier column is not the first column

- where the identifier column in the workings spreadsheet is not in alphabetical order

I need to understand the consequences of each of the above scenarios pls.

Thx.