When I try to modify the assigned resource's actual labor hours of a task by using XOG, the result is always "success" but the Actuals are not updated.
Initially, this was controlled only through the Time Tracking mode of the project/assignments.
If the track mode was Clarity (or PPM) then the only Actuals that may be put onto this task/assignment would be through PPM's timesheeting and updating the project through XOG would not work.
To overcome this, in pre-14.x PPM versions it was possible to temporarily change the Track Mode to another value like 'Other' and 'None' and then to update Actuals on a project using XOG.
Now an additional check was added to see if the original assignment was done via timesheet - which means that the Track Mode was previously Clarity / PPM.
If so, then the updates to the Actuals via other mechanisms like XOG will be blocked.
This behavior has changed to avoid possible integrity problems on Actuals, the only solution is to use PPM's timesheeting capabilities.
Note that when the original Actuals are created by XOG, they can be updated by XOG.
However, some time ago it was noticed this was being 'abused' (project managers could influence their project inappropriately for reporting or consistency purposes) and so an additional check was also made to see if the assignment had previously appeared on timesheets - which would only have happened if the Track Mode was previously Clarity / PPM. If so, then the updates to the Actuals via other mechanisms will still be blocked.