Creatin g formulas that reference cells on multiple sheets

I have been creating a work book which has two sheets. One sheet has monthly targets and in the other I am creating a sheet that i can add monthly invoicing figures to and then using a formula that uses the monthly target from the other sheet and the cell where I input the invoicing to tell me monthly shortfall.
I have recently done the intermediate excel 2007 course and have been using the notes on page 14 of the manual.
The formaulas have all been calculating correctly but now half way throught the sheet they are not working - not sure what I am doing wrong?

thanks,

esmee