When you add a teleconverter, it not only magnifies or adds focal
length, it also decreases the effective aperture of the lense by a
relative amount, increasing the f-stop of the lense.

The smaller effective aperture reduces the amount of light that hits the
autofocus sensors, and if the light level drops too much, the camera can
not focus properly - just as if you were trying to focus in dim light.

With most cameras, the critical level for autofocus is f/5.6 or better.
Some pro level bodies can autofocus with a bit less light.

This is typical behaviour.

A 2x teleconverter drops light transmission by 2 stops. So an f/2.8
lense acts like an f/5.6 and a camera body can still autofocus just
fine. But if you have an f/4 and add a 2x, then the light drops off too
much and you get an effective f/8 aperture - not enough light for the
autofocus sensors.

If you use a 1.4x teleconverter, which adds 1 stop, to an f/4 lense, you
get an effective f/5.6 so once again it still focuses ok.

<snip>

That's true for maintaining auto-focus, but there can be physical
limitations as well. I have the Sigma 1.4x and the 2x, that I normally
use with the Sigma 70-200. I believe that its my Canon 100-300 that
needs to a 12mm extension tube between them so that the rear element of
the Canon lens doesn't hit the front element of the Sigma converter (it
moves in and out as you zoom). Technically, they would be termed
incompatible. They work optically - it's teh physical part that is the
trouble.