Sales Tax Code Missing

We are trying to process an AR invoice that was created from a packing
slip. The invoice has 4 lines on it and everything worked fine for the
first 3 lines but on the 4th line when we try to select MN 6.5% it
doesn't even show in the drop down list. We looked through the lines
for any kind of differences and couldn't find any. Has anyone else run
into this?