It is most likely a user error. You need to know what you are doing. It is absolutely impossible not to achieve a manual focus with an extension tube. There is no magic there, no glass inside. Just put it on, use the manual focusing and focus using the back screen on your camera (a.k.a. live view). The manual (or internet resources) will tell you what the optimum focusing distance would be for each lens (and focal length - usually above 50mm). Put the subject within that range and try. The depth of field for macro photography is razor thin. So, if the subject is not in that range, you can't focus, of course. Once you are able to determine the optimal range, you can try AF, it should work too.