Check:
- Auto Join setting -- enabled by default
-Country Code - look at the part number sticker on the back on the IAP's. They all need have the same country code designator (-US, -JP, -IL, or no country code which = Rest of World).
- Does the site have internet access? If not, and the new IAPs are running a different version of code than the VC they won't upgrade and won't join the group.
Try connecting a console cable to the console port of the new IAPs and watch/capture the boot process.