It is only possible to encourage a connection that is on the same challenge day as you. This means it can get a bit tricky when trying to encourage a connection that is several time zones away.
As an example...
Kate lives in Oregon, and Mary lives in Uganda.
When Mary's challenge for Tuesday has been issued at 7am in Uganda, it will still be several hours before Kate is dealt her Tuesday challenge at 7am in Oregon.
In the hours before Kate's challenge is issued (putting the two connections on the same challenge day) it is likely that Mary will complete her challenge and thus cannot be encouraged by Kate.