If you have the same type of APs, as you mention, please check:
- Are the new APs of the same type and regional code, check the sticker on the back which should show something like AP-505-RW / AP-505-US, or AP-375-RW / AP-375-US; as well the suffix should match between the 505 and 375.
- Do you have the AP autojoin enabled on the VC (enabled by default)
- Check the AP console logs on the VC and the joining AP for an indication why they won't join. The AP-375 can use a standard micro-USB cable to access the console, so I would check those first.
If that doesn't bring any light, I would contact Aruba TAC Support to have a look together.