Check the configuration in Main Menu > Diameter > Configuration > Egress Throttle Groups to determine if the Maximum Configured rate is too low.
Check the Egress Message Rate at Main Menu > Diameter > Maintenance > Egress Throttle Groups and Main Menu > Diameter > Maintenance > Connections to determine if the sending Peers/Connections are offering too much traffic.
Determine if the receiving Peers or Connections in the ETG are not responding with Answers in a timely manner because they are either busy or overloaded.