40 D Back Focus Problem

I have had the 40 D for some time and have been back focusing using the AF-ON button for most of the time. I know it is set by using the C.Fn IV-1 setting. But now, for some reason, the shutter button is in control and the AF-ON does nothing. I know I must have changed some other setting that affects this, but I can not figure out what is going on. I am in Av, AI Focus (all thought this setting change does not seem to matter). Does not seem to matter what lens I use either. Any ideas? Thanks

What mode are you in? Not sure about this one but some items only work if you are in Creative(AV, TS, M etc).

Bill

AV mode. I agree, it does not work in all modes. I use AV most of the time.

if in doubt you could just return it to the default settings, via the reset(clear all of you settings, and return to original)
Don

I ended up doing just that, I reset to default settings and it seems to have fixed it. I will see how it goes these next few days. Thanks for the help.