VLOOKUP Inconsistently Returns Formula Calculated Cells

I'm having a real problem with the Vlookup formula in that I have used it in a spreadsheet to search, using approximate, rather than exact values, several tables where I have inserted raw data with no calculations and it works fine. But with one crucial series of cells where I am trying to return results from columns of fields that are the results of calculations, it sometimes returns the correct result and other times returns an empty field with seemingly no consistency at all.

I've been careful to make sure the data in the cells in the lookup column are in ascending order, as required. I've checked the formatting (all the lookup cells are numbers, no zeros with the thousands commas inserted, and all the return cells are currency with two zeros and the $sign) and the formatting is consistent.

Does anyone have a clue whether the calculations in the cells might be the problem or if it might be something else and, whatever the case, how I can workaround the problem?

I'm a relative novice with Excel. I'm using Version 2007 but I'm saving the file as an .xls.