Try using a Raw/Plaintext UDP input instead of a Syslog UDP input.
Also make sure that all devices are configured correctly and send their syslog messages to port 1025/udp of your Graylog node(s).
Does it work when sending a hand-crafted syslog message to Graylog?
# echo "<0>1 $(date --iso-8601=seconds -u) example.com - - - - test message" | nc -u -w0 graylog.example.com 1025"