Logitech mouse used on El Capitan
If you look at System Prefs > Displays  > Arrangement, does it look like this?



or more like this?



If it looks more like the latter, then you will only be able to drag the cursor from one screen to another at the point they intersect on that picture. Trying to cross the border anywhere else will not work.

Also make sure that the arrangement in the picture is an actual representation of your physical layout &amp; not, for instance that your secondary screen is represented at the wrong side of the primary. If so you can just drag it to a better position.
Clicking one of the monitors on the control panel will edge it in red, on the picture &amp; also your actual screen itself, so you know for certain which you are moving. Try to line them up to the best physical representation you can.