'Volatile' OFFSET function not giving correct answer

[Michelle]

Hello, I have a formula that looks at some parameters, and then returns a
value from one month or two months previously, it works most of the time,
but occasionally just doesn't. I've followed it through and used watches and
everything I can think of, but can't work out why.

the formula looks like this:
=IF(OR(MONTH(AF$7)=properties!$B$16+properties!$B$15,MONTH(AF$7)=properties!$B$16+properties!$B$15+3,MONTH(AF$7)=properties!$B$16+properties!$B$15+6,MONTH(AF$7)=properties!$B$16+properties!$B$15+9),IF(OFFSET(AF$1042,0,-properties!$B$16)<0,-OFFSET(AF$1042,0,-properties!$B$16),0),0)

But the first IF clearly equates to false, so it's really only using
IF(OFFSET(AF$1042,0,-properties!$B$16)<0,-OFFSET(AF$1042,0,-properties!$B$16),0)

the formula box displays 'Volatile' as the result of the formula but the
values are as follows:
AD1042 = -60 (this is the cell 2 to the left of AF1042)
Properties!B16 = 2

so by my calculation, the formula should return '60'. But it is returning a
zero

I'd love to find out why this is happening - it's driving me nuts!

Thanks

M