Just after a workstation is deployed with MDT a bundle is launched after a refresh. This bundle contains an action "Install bundles" at the end.

The workflow is working well.. but not always.

In some cases (20%-30%) of time, the first bundle is downloaded at 100% but nothing else happens.
The technician must manually type "zac ref". This is working in much situation but sometimes not. In this case he has to go to manually run the bundle.
I made and action to run a zac ref just after downloading (in Distribute Tab) but it did not help.

Any idea about the cause and a solution?

ZCM 11.2.3a, several primary servers, Windows 2008 R2