If already running SMs are working fine and registering with AP while only newly introduced SMs to the network are having problem then please check if any is configured under Security TAB of AP or not.
If Authentication Mode is AP Pre-Shared Key then check if using Default Key or manually configured key (use Key above). If key is set up manually then new SMs must have to have the same key in Security tab.
Go to SM GUI <i>Configuration --> Security_ --> Select Key "Use Key above" and write same key in Authentication Key box which is configured on AP.