I hate when my apps doesn't show up in the launcher too 😒😒
I think AppEditor can solve your problems, it's a great app for editing our launcher/slingshot entries. Link: AppEditor
I will try to show you a practical example, hope it can help.
Goal: Create an icon to launch the Python Interpreter. As you can see I have no Python Interpreter in my Programming Section.
Step 1. Launch AppEditor.
As we can see there's no Python in Programming here as well:
Step 2. Activate -> Show hidden entries
Wow! Look who's here. Python it self! I will activate the 3.5
Step 3. Activate -> Show in Launcher. Don't forget to Save
Now let's check if it works.
That's it! You can play and change the categories and even edit the myapp.desktop file directly. In most cases this will work, of course we'll find some exceptions.