I have 3 x One SLs in different rooms around the house. Bought 2 x Era 100s at the start of the year to add to the system as the One SL’s no longer in production. Set them up and added to the system, but they keep disappearing. 
 

Have re-booted the system and looked re-added but the same thing seems to happen. It’s only the 2 x Era 100s which disappear. 
 

Ubiquiti WiFi infrastructure and good connection throughout. 
 

Any thoughts welcome. Thanks