There are a number of potential reasons. It is hard without examples and testing to isolate cause to know, but here are some things you can consider and try.

First, is the F8 result of adding the TC as noted in another answer, even if F8 is acceptable to your camera, it is pushing the limits, and auto-focus will be less reliable. Testing with live view focus (if Canon has it, i.e. focusing through the imaging sensor not the AF system) can help find out if you are having AF issues.

Secondly, also as noted, TC's tend to work best with primes and wider apertures. The optical complexity of many zooms, especially wide range zooms, already have some compromise of quality to achieve those zoom ranges. Adding more magnification will exaggerate those compromises. That is not to say they cannot be used, just that you are adding to sources of degradation together. Checking for reviews of your specific combination to see how they work in the real world may help you determine if you have a bad combination, or something else. Also consider shooting without filters and see if that matters (hopefully not, but experimentation is what you need).

If you have AF Fine Tune on your camera, it may be necessary (or at least better) to fine tune. At least on Nikon and I think Canon, fine tune settings from the lens alone are not applied to the combination -- you have to tune that separately. I do not mean to imply this is the cause -- at F8 you have a fairly deep DOF already. But many people who do fine tune do not realize they must do it again with each TC combination.

Long focal lengths require a lot more stability. An oft quoted rule of thumb is the reciprocal of the focal length (without VR/IS), but I find as you get higher resolution cameras (where people zoom 1:1) and very long focal length, you really need more than that. Maybe much more. I personally find even at 4000th of a second I get camera shake at 800mm shooting baseball, and really had to work on my monopod technique. I finally convinced myself it was me by shooting a game with a tripod from center field. The effectiveness of VR/IS is highly dependant on the implementation, as well as on your handholding technique. And on shutter speed, generally it is ineffective entirely as you get over 500th of a second, and on some systems may actually do harm, so turn off IS/VR if shooting fast to see if things improve. To determine if camera shake is the issue, get a stable tripod and shoot the same scenes in (otherwise) the same scenarios.

It is also often possible to separate motion blur from AF issues by zooming close and examining the whole image. If you find there are areas that are sharp -- but not on your subject instead behind or in front -- your issue is likely focus. Look for that DOF range in ground, trees, etc. that have texture. If you find the entire image is soft, it is more likely camera shake. Sometimes you can confirm by finding point sources, sharp lines and the like, and see if there is a direction to their blur, indicating motion. Missed focus will tend to be circular, motion will often show direction.

Finally, and related to the stability issues, some TC's tend to have a fair amount of slop in their mounts. If you find you can twist the lens a few degrees that is not so bad, but if you find there is any significant play side-to-side, up-and-down that allows the lens to get out of a perfectly straight alignment, you might need to get service.

And do not feel that there is a specific single case - these type of issues tend to be additive, all contributing a bit. You need to consider that you can suffer a bit from all such possibilities (and maybe some I have neglected). As you get to 500, 800, 1000mm you are getting into a very demanding range, especially with high resolution cameras that let one see every flaw. You need to control all such issues to get crisp, sharp results.

Linwood