I have a working link between three MikroTik STX radios running on 5 GHz.. The link is currently configured for a bridged network. One of the radios is configured as ap-bridge and the other two are configured as station-bridges. All other radio settings are identical including the WDS settings of dynamic (with a bridge interface). I can successfully pass traffic between all the devices using either NV2 or nstream. (I don't use 802.11 because the distance between the ap and the clients is over 4 km and the link does not work well with 802.11.)

I can communicate between either of the clients as well as between the Ethernet ports of all the radios as desired.

I want to make one of the client radios into a second ap-bridge so other clients can connect to it as well, but I'm unable to establish a link between the existing ap-bridge radio and the client that I want to "promote" to also be an additional ap-bridge. When I change the role of the client from station-bridge to ap-bridge, no association takes place regardless of the protocol I attempt to use (nv2 or nstream). I see no errors in the logs, I simply get no association.

Any suggestions how to diagnose this to get it working would be greatly appreciated.

In both nv2 and nstreme there must be a controller and managed device. So in this case you can have only 1 ap-bridge and all clients on that link can only be stations (station, station-wds, station-bridge).
What you want to do ap-bridge <-> ap-bridge can only be done in 802.11

Cybertod, thanks. I did not notice this fact elsewhere. Most unfortunate.