Check which layer has the active-layer icon (the pencil) for your canvas in the Canvases panel. It indicates which layer is active for editing (including pasting). If you've just created or worked on the shared layer, that's where it will be. To change the active layer, carefuly click in the status area to the left of a layer rectangle in the Canvases panel. You can also move the pencil by double-clicking a layer or by using the gear-wheel menu at the bottom of the Canvases panel.
It's a little confusing, but the logic seems to be:
Selecting a layer is done by clicking on the layer anywhere or by using arrow keys. Menu commands (Delete, Duplicate, etc.) act on the selected layer.
Setting the active layer is done by clicking in the left-hand portion of a layer, double-clicking anywhere on it, or by menu. This also selects the layer.
Selecting a layer without making it active is done by clicking on it outside the left-hand status area.
Last edited by john.gersh; 2012-04-04 at 07:08 AM..