Multiple IF and vlookup statements excel 2003

I am using 6 IF vlookup statements across 7 workbooks. The Formula is =if(A1=vlookup(A1,Sheet1!A:A,1),"Sheet 1",if( copied 6 times with the only thing changing being the sheet the vlookup looks in and the result if true. The problem I'm having is cells that should be returning a Sheets name (Sheets 4,5,6) are returning #N/A for the earlier numbers and working fine for the later numbers. I have checked the numbers and they are the same, even copied and pasted the cell to make sure. Rewrote the formula in another cell and the same thing happens.