I've printed the debug log to the console and looked for clues, it says:
CActiveMasternode::ManageStatus() - Checking inbound connection to 'xxx:yyy'
CActiveMasternode::GetMasterNodeVin - Could not locate specified vin from possible list
CActiveMasternode::ManageStatus() - Could not find suitable coins!
CActiveMasternode::Dseep() - Error: masternode is not in a running status
CActiveMasternode::ManageStatus() - Error on Ping: masternode is not in a running status
But I've started the nodes, they are synced, and in the masternode list.. But I think they'll all drop out?
masternode start-alias mn_name your_password