the lookup_value and the first column of table_array

Should both be number or both be text.  Here is the note from Excel Help:

• When searching text values in the first column of table_array, ensure that the data in the first column of table_array does not contain leading spaces, trailing spaces, inconsistent use of straight ( ' or " ) and curly ( ‘ or “) quotation marks, or nonprinting characters. In these cases, VLOOKUP might return an incorrect or unexpected value. 

You can use either CLEAN function or TRIM function to clean the data up.