Excel Vlookup Function

I have another issue in which the vlookup function returns a wrong value, no matter what i do i get the same outcome. For example i have a lookup table with the commissions of banks and the performance grade for these commissions. Say one of my banks had a commission of 600,000 and the performance grade would be B. Using the vlookup function it returns F which means $0.00 commission which is not the case. Please help