why my XL1S often out-of-focus ?

Thx

Do you shoot in Manual Mode?
Is your lens set on manual too?

Know that the XL1s has a pretty soft picture in comparison with other DV cams, and that the Viewfinder sometimes is not good enough for accurate focusing (altough I myself never had problems with it).
You can also use the PUSH AUTOFOCUS button if you use the lens manually.

It could of course be another problem, but in that case, I don't know anything about it, so I'll leave it to other people to answer further.

First make sure your viewfinder is sharp. Focus in telephoto on a far object, do this in manual focus in any mode except for easy mode. Then move your viewfinder back and forth until the picture is clear in the viewfinder.

Next, make sure you sharpness is up in your camera. It should at least be in the middle position.

You may have to send your lens in to Canon if you can't get it to focus.

Also, the auto focus need a reasonable light level to work well, (the Xl1 auto focus was rated for around 50 lux) and the image needs vertical contrast; e.g., edges.

Especially with older or use gear, if the poor focus is only at certain zoom settings, you might have a back focus setting issue (not very common) and with the stock lens that is an internal factory setting.