QuickSupport issue for some remote users does not work with service sessions

I have customers (users) that I support remotely. I thought the new service sessions with the quick support were nice, but now I run into a problem.

For some users the quicksupport tool that uses service sessions (both the generic one ánd my customized one) fail to show up as "online" in my TeamViewer application (while both me and the customer are "ready to connect"). As a result I cannot build a connection with them.

If the SAME customer on the SAME machine uses the QuickSupport version that gives a number and a password, I can connect to them without any problem.

Why does the service session version not work? It would be so much more convenient.