Forum

I have a fresh 3.8.1 install on a dedicated server. I am running 

php 7.1.33 with
max_execution_time=480
memory_limit=640M
post_max_size=128m

I have a forum with approximately 200 users (all currently subscribed). However when a new discussion topic is posted it is delivered to an
inconsistent set of users. For example a posting early in the day will be delivered to users from certain domains yet those exact 
same users will not receive a post to the same forum just a few minutes later while other users on those same domains will receive 
both the posts. 

I have spent the better part of 2 days researching this and have not seen anyone with a similar problem or solution. Any ideas?