VLookup returns N/A or wrong value

hey there,
I am working on an excel sheet with around 21,000 rows and using VLOOKUP function to compare two worksheets and get a common value. I need to lookup the column C (Long code print format) in sheet one and compare it to sheet 2 to find the amount corresponding. The results are displayed in Sheet 1 column D. I have attached a dummy sheet whereby if i use the formula =VLOOKUP(C:C,Sheet2!A:C,3,FALSE) it works like a charm and gives me the correct amount, but if i implement it on the whole sheet, it gives me a N/A result.

I think its also worth noting that if i use TRUE in the function, it gives me a result that is a row above what is expected. What could be the reason behind that?