I don't know for sure which camera you have, so I can't get too detailed
Canon A1100 IS
Canon SD1100 IS
Most likely cause is that you have "Digital Zoom" enabled.
Digital Zoom should be turned off or disabled.
Digital Zoom degrades your picture.
In case you have something set unintentionally I suggest you also do a "Reset All"
Look at the Index in the back of the User Manual and look for "Reset All".
That resets the camera to factory settings.
You should check Digital Zoom again because "reset all" might turn-on the digital zoom.
Turn-off digital zoom.
Canon Powershot 1100IS takes excellent pictures except if I zoom, then it is out of focus.
Even though it is older, heavy, and dated, it is too good to give up on if there is a setting or something I may have wrong creating the zoom focus problem.