Could be any number of things, here are a few solutions I've come across in the past.
1- Try retyping the data in 1 cell of the spreadsheet, saving and then have monarch refresh the lookup. Even though I may change the format of a column in excel the actual cell format doesn't seem to change until I activate the cell and enter out of it again.
2- Make sure there's not any leading/trailing spaces on the lookup value (both in excel and monarch)
3- If data has been added to the original excel document be sure the range is including any rows or columns that have been added.