Excel 2003 SP3

User formula is =VLOOKUP($A30,'ATS '!$A$2:$B$91,2).

In sheet ATS she has data ending on row 74. She has allowed 91 rows to
include new entries as needed in the future. However, the formula works
intermittantly. For some entries she has to change the formula to $B$B80
(give or take a few rows). Is there a limit of blank rows that can be used
in the lookup formula?