IFERROR (VLOOKUP) on a concatenated field

In a Numbers 09 file, I need to find student records in two tables that have identical values in two specific columns and discrepant values in a specific third column.

Because of difficulties getting an IFERROR (VLOOKUP) formula to function properly, I tried to simplify by concatenating the three columns in a new column in both tables using the formula

=CONCATENATE(A2,S2,T2)

then copy-pasting as values, and making sure the column's cell format was Text.

The following formula works properly.

=VLOOKUP(U2,TABLE1 :: $U$2:$U$2582,1,0)

where column U in Tables 1 and 2 contains concatenated data from the three columns of interest. This formula returns correct values for records that match between the two tables' concatenated column, and returns an error for records that do not match; I determined this by spot checking records in the two tables (Table 2 has almost 1500 records; Table 1 almost 2600).

The following formula, under identical conditions, returns "Not found" for every record.

=IFERROR(VLOOKUP(U2,TABLE1 :: $A$2:$U$2582,21,FALSE),"Not found")

where 21 is the column U containing concatenated values in Table 1.

If I copy-paste cell U2's value in Table 2 to the same cell in Table 1, the IFERROR forumla in Table 2 still returns "Not found."

How come the above VLOOKUP formula returns correct values while the IFERROR formula does not? What am I doing improperly?

Thanks in advance!