Swarm node Status down, but node should be Ready

Hello everyone, I am trying to run a service on a swarm composed of three Raspberry PIs. I have one manager and two worker nodes. The problem is that sometimes the status of the worker nodes is “Down” even if the nodes are correctly switched on and connected to the network.
I just started using Docker so I might be doing something wrong, but everything seems to be correctly set.
Any help would be really appreciated.