The RIGHT function will return a text value, although your customer number IDs are actually real numbers. You can overcome this by changing the formula in H10 to this:

=VLOOKUP(G10+0,'Sales Reps'!$A$2:$B$7,2,FALSE)

That +0 will change G10 to a numerical value, so the VLOOKUP will work correctly. Note that I have also anchored the range by using $ symbols, so you can copy this down without the range changing.

Hope this helps.

Pete