If you have the kind of stretcher bars with fingers on the end, those are designed to fit together in a loose manner such that they can be later expanded by adjusting the triangular shaped wedges that usually come with that type of bar. Canvas tends to sag over time, which is why you need the adjustment.
Search "stretcher bar wedges" and look at the images.
The other school of canvas bars uses basically a glued and/or stapled together, rigid frame. In theory, a quality canvas properly stretched at the get-go will stay stretched forever. Yeah right. If that kind of canvas should sag, the only way to tighten it up is to remove the staples and re stretch.
PS. The initial stretch is much easier on the rigid kind of frame.
All of which is why I glue my canvases to Gatorfoam board.