Your problem is most likely in the module definitions for the module. You must have one of the controls listed as a control type of 'view'.
In general, to display a usercontrol specifically (and no other modules on the page), you request under the Url format of:
example.com/pagename/tabid/xx/ctl/ctlkey/mid/yyy/default.aspx
Where:
xx is the tabid
yy is the module id
ctlkey is the key of the control as configured in the module control definition.
Each DNN module has a type of 'view' which means the /ctl/ctlkey/mid/yyy does not need to be included in the Url - so a standard url of example.com/pagename/tabid/xx/defualt.aspx will show all modules on the page, specifically the controls that have a type of 'view'. Because of this, the 'view' type doesn't have an associated key.
So, to solve your problem, go to your module definitions page, click on the module, then check you have a 'view' type control listed, and that the view type points to an actual module control.
Finally, don't forget it might be working OK but your module isn't actually producing any valid Html. This can be checked by putting a breakpoint in the page load for the control and seeing if it gets triggered.