Sorting values in a lookup field

Sorting values in a lookup field

I've created a job db which uses job numbers. I have a number of forms that link to the job using the job number. I have set the look-up to sort, but in some forms it's not sorting as would expect - see example below:

Job Numbers:

J016
J007
J026
J019
J030
J998
J999
J040
and so on...

Some forms do sort the job numbers correctly - any ideas?

Martyn