can you try to check your AWS settings (and logs) of hosts for those members? maybe there was a system update (hence a reboot) that broke some settings. or maybe someone tried to update mongodb but did not follow clean steps.
the “log file” set in the config should be showing what errors are encountered when mongod tries to start. if it gets an error, it will just exit to keep data safe meaning there won’t be a service listening on port 27017. this also makes it easy to search the log file because the error won’t be far away from the last line logged.