I haven't seen an AGP card yet that wasn't backwards compatible.
I would take the card out and then re-seat, make sure it is pressed in.
If you have another system, try the card in that PC to see if it works.
If that doesn't work, and the card is still (fairly new) I would RMA (basically return it and get another one through the vendor).
If it does work in the other system, mobo, whatever the case, then I would suggest that maybe your new mobo's AGP is dead.
You could say I've been around.