STP between 2824 and 6224

I am trying to get STP working between a 2824 and a 6248.

The 2824 mac address is lower, so when the priority is the same, the 2824 configures as the root, and everything works.

However, when the priority on the 6224 is lower, both switches configure as root.  It looks like the 2824 does not see the BPDUs from the 6248. When both switches configure as Root Bridge, the 6248 puts the attached port in discarding state.

I've checked through the debugging stp: duplex is ok, the stp mode is rstp for both, parameters look OK, I've got the port mirrored and can see the BPDUs going back and forth, and for the life of me, it looks like the 2824 is just ignoring the BPDUs coming in from the 6248.

I don't see any protocol counters, or any obvious way to see inside the 2824 (show tech-support doesn't help a lot).

What am I missing here?

Thanks!