There are a bunch of reasons why an IAP would not join a cluster:
1) AP does not get an IP address (Not your case)
2) AP's country code does not match with that of the existing clusters'
3) Image version does not match
4) If AP has been configured as Standalone AP or Prefered master (While there is another preferred master existing)
In your case its mostly the image version miss match as you are suspecting.
To address the same, the options are:
1) Put them in a separate VLAN where no other IAP existing and upgarde
2) Other option is to do an upgrade from the existing VLAN by getting into apboot (need to mention tftp server and change the boot mode to tftp) - but this method is difficult for doing for 10 APs. Easier is the first option.