User Statics is enabled, but hostnames doesn't appear sometimes in the hosts lists.

We have recently enabled the user stat variable for all MySQL hosts, Initially few hosts names did not appear in the list, later we re-installed the pmm-admin for those hosts and was able to get all the host names in the list. 
But now we gave the same issue, the host name are not listed even the pmm-admin was installed on the host and connected to the pmm-server.  

Here’s the output from the prometheus for the missing hosts the error is out of order sample”.

How many hosts do you have ?   You can see some of the “lr” scrapers are down it is possible they are timing out because getting information takes too long.   This often can happen when you have large number of objects.