With support meeting, the expectation is that the users desktop will automatically be shared and you will have control. What message does the user joining see?
Do you see any messages stating failure on your side?
Do you have remote control enabled on the config (I am guessing yes, but wanted to confirm)?
Does the same behavior occur with non-support meeting types (this will require you to give presenter status to the user, sharing the desktop, and then you requesting control; or give user presenter status and then you request control and the user will have to accept and choose what to share)?