If you have sufficient hardware to support the number of VDOMs you are running, check the global resources on your FortiGate unit. At a glance it will tell you if you are running out of a particular resource such as sessions, or users. If this is the case, you can then check your VDOMs to see if one particular VDOM is using more than its share of resources. If that is the case you can change the resource settings to allow that VDOM (or those VDOMs) fewer resources and in turn allow the other VDOMs access to those resources.