Can you activate that option, drag lines in the sketch and then deactivate that option. Also restart SW or your machine and check.
Tools Sketch Settings > Overide Dims on Drag Move maybe or the Sketch Auto Solve mode being deactivated might be issuesome. I've had problems with SolidWorks thinking I want to dettach in similar situations. Maybe Shift being held down or some other modifier it could cause issues so check your kb buttons.