Currently OmemoConfiguration#IGNORE_STALE_DEVICES is default to true.
If two OmemoDevices have not been sending omemo messages for the last 7 days to each other, it is not further possible for either of them to unlock the IgnoreStaleDevices state.
There is also no feedback to the application upper level when this happened; so as to allow app to prompt user to take the appropriate action.
Alternatively Omemo may intelligently recover from this situation. otherwise this option offers little benefit to user. May be the lastMessageDate should get updated when an Omemo message has been received without the need for it be decrypted first. But no sure if this has breached any security.