Sounds like your network pool is busted. If you are using an vApp, Org Routed, or Org Isolated Network ... it sounds like the transport between the two doesn't work.
If the VMs are on one of those network types and on the same host, it works. If they are on different hosts, it fails.
What type of network pool is this? VCNI, VXLAN etc?
Make sure that the physical network is actually configured to transport the data. e.g., VCNI uses a VLAN and MTU of 1524 (suggested min) to transport between hosts. However, you actually need to configure this on the network fabric (switch, etc).
VXLAN has its own requirements, but has a way to test the virtual wire.