Hello, I'm working with Excel and having problems with Vlookup...the function vlookup is returning "N/A" when the value exists. This error occurs only sporadically. For some rows the value returns,...

Hello, I'm working with Excel and having problems with Vlookup...the function vlookup is returning "N/A" when the value exists.  This error occurs only sporadically.  For some rows the value returns, for others it doesn't.  What could be causing this error to occur?