I find it somewhat hard to fluently work with symbols in Mockups 3, especially when in Symbols view. Here’s one thing I’ve noticed. This is with 3.1.8 on OS X 10.10.4.
If I right-click on a particular symbol or symbol library to call an action on it (e.g. “rename…”), the selection stays with the currently selected item, which makes me wonder, if the action is called on the selected item or the item I’m hovering on. In the image below, the orange arrow points to where my mouse pointer was, when I made the screenshot.
I suggest you either move the selection to the item below the mouse pointer once I right-click. Or you introduce a focused item (see screenshot from Apple Mail below). I favor the second option as it preserves the selection, but also allows for better keyboard support, in that it would also allow you to eventually support multi-selecting items using the keyboard.