Postgres RDS Cant increase IOPS

What db-instance-class are you using? There are also IOPS limits described on the ec2 instance type. You might need a larger instance type to support very high IOPS.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-optimized.html

yes above link shows max and baseline IOPS/throughput limit for each EC2 instance type. Remember to consider baseline IOPS/throughput limit for all instance type which has * as those can only support up to 30 min (in 24 hour) of max IOPS/throughput limit...Its also mentioned under first table in the same article. As you can see db.r5.large can only support baseline IOPS of 3,600 but max IOPS is 18,750 ...so you may get up to that max limit once up to 30 min in 24 hours windows...after that we will throttle it at 3,600 total (read+write) IOPS...I hope that explain it..

Im using db.r5.large. What i think is odd is that i have a similar instance that runs with 6500 just fine.