vfs.fs.size shows no value for some clients

[awrow]

I try to get free disk space for an Ubuntu 18 machine.
An output from df command is in attachments.
A key value I use in an item is

vfs.fs.size[/,free]

It works on other machines but not on that one. On the "Latest data" tab for this machine the name of the item is gray and shows no value.
How can I get the free disk space for that machine?

[awrow]

In the "Latest data" the item showed nothing for an hour but now suddenly!! it shows the correct values..... don't know what the problem was... magic