Vlookup formula responds selectively for some and not all values

Hello,
=IF(G88="PT",VLOOKUP(B88,Vlookup1!A:R,5),IF(G88="OT",VLOOKUP(B88,Vlookup1!A:R,6),IF(G88="ST",VLOOKUP(B88,Vlookup1!A:R,7),FALSE)))

Two tabs in an excel spreadsheet. One with this formula and another one named Vlookup1 with the info (three columns indicating diagnosis codes for PT (Physical Therapy), OT (Occupational Therapy) and ST (Speech Therapy).

Out of approximately 350 rows, this formula works perfectly fine with all but three customers’ names. It seems to be only with the last three customers –sorted alphabetically– and can’t figure out how to solve it.
Have reduced the amount of rows in the info tab / have changed the format of all cells for these 3 customers / have retyped all information under another row and inserted above all the conflict rows but it always brings “0” as the cell value – not even False….

Help!!