Using this formual to find enter a tracking number to an order number.

=VLOOKUP(A2,Sheet2!$A$2:$B$287,2,FALSE) where A2 is the order number and
Sheet 2 A and B are the order numbers and tracking numbers

The problem is, it gives me the correct answer for the first cell and when I
copy it down it just repeats that same number? If I actually click on the
formual for the next cell and then hit enter (not changing anything) it then
returns the correct tracking number? Do I have this fomated incorrectly?
Any help would be appreciated, otherwise I need to click on 2000 formuals
and hit enter.