I think I know what is happening for you. As I add support for new sites to Sourcer there can be more Search menu items on the Sourcer menu (it depends on the country and timeframe of the source on the current page). Since there is a maximum height for an extension popup window this can cause the menu items below the search items to fall off the menu. You should be able to scroll down in the menu to see them.
When this happens for me I see scroll bars on the menu. I can use those to scroll down, or I can use the mouse wheel or the keyboard up/down arrow keys.
There are also options to control which search menu items show up. So, for example, if you don't have a FindMyPast subscription, you can set that not to show up.
I described some of these options in the WikiTree Day presentation I did earlier this year - available on YouTube here.
The options to control the search menu items are all under the "Search" tab in the WikiTree Sourcer options. You can control if and where the site shows up in two places 1. the main Sourcer menu. 2. The search submenu reached via the "Show All Search Sites..." menu item.
For each supported site you can set the priority on each of these menus. A priority of zero means this site is not shown at all. A priority of 1 is the highest priority and puts the item higher up in the menu. A higher number puts it lower down.
If you wish to you can set it up so that only your favorite sites are on the main menu and you can get to the other sites via the submenu.
You can also set the maximum number of search menu items that will ever be shown on the main menu - this can stop any of the citation menu items being pushed off the bottom.
