Two issues I have found in the current mbarcode: 1) It is not quite clear when mbarcode actually makes a shot. More than once I continued feverishly clicking the shutter button.
2) Once you click on one of the plugins, the application seemingly hangs, and may continue in this state for a minute or so (while it is searching for your barcode). It should show the "busy" status instead (rotating thingie in the menu bar may do).