inset not working

Hello. Can you can give a little more context to the problem? It can be a number of things, so to really come up with the solution it's easier to see the problem. If you can describe more about the problem add a screenshot or even a recording screen capture. 

Hi vi SSuperNova ,

It is possible that you accidentally pressed I twice, the first one is to activate the Inset tool and the following times it toggles Individual off/on:

Try Pressing I again and see what happens.

The other possibility is, like already mentioned above by Krzysztof UUserData , that you have 'double Vertices'. Try Merging.

Hi vi,

please check your Inset settings:

Yes there can be such a thing as inseting too much. You can go past a threshold and end up overlapping your faces. Just get the inset until you reach a point where the faces don't overlap, then you can arrange the cartridge slot to your liking by arranging the vertices.

For example, you can select a row of vertices and scale them to zero in the X o Y axis by hitting on your keyboard, S to scale, followed by the axis, so Y or X and then zero and it will align those vertices. That way you can arrange you slot. 

You can always upload your file and give us the link if you want us to take a look at it.