Eyo,
This is a replication!
Link:
Make sure that your Player Character & HealthComponent is replicating by checking all the boxes in the constructors. I advise you to do an OnRep_Event instead of Multicast as this sure that the information will get through.
Here is my go-to Link when I need a reminder:
A new, community-hosted Unreal Engine Wiki - Announcements and Releases - Unreal Engine Forums,Using_ReplicatedUsing/_RepNotify_vars