Vlookup problem, gets stuck halfway through looking up table

Hi I am having a issue with vlookup, any help would be much appreciated:

I am trying to match accounts with a ref for example

I paste in acount 123456789 and i use the formula vlookup(AI,accountref;2)

I then have a seperate sheet were I list the accounts and next to the i have the ref
For example

A b

123456789 ref1

134567892 ref1


145678923 ref2 etc.
This usually works very well and gives me what I am looking for however,

I have noticed that the lookup get's stuck about halfway down the list, there are no duplicates and the format are the same, eg.

i paste in account 145678923 and I get ref1, for some reason it thinks 145678923 is 134567892

Any help would be much appreciated

Kind regards

Try adding a zero as the fourth arguement to force an exact look up.

=vlookup(A1,accountref,2,0)

Are you sure that the values are identical in the two lists? Check for trailing spaces.