Docker Swarm Overlay Network Not Working Between Nodes

i am trying to connect my docker services together in docker swarm.

the network is made of 2 raspberry pi’s.

i can create an overlay network called test-overlay and i can see that services on either raspberry pi node can connect to the network.

my problem:

i cannot link to services between nodes with the overlay network.

given the following configuration of nodes and services, service1 can use the address http://service2 to connect to service2. but it does NOT work for http://service3. however http://service3 is accessible from service4.

node1:
  - service1
  - service2
node2:
  - service3
  - service4

i am new to docker swarm and any help is appreciated.