Resolve uses source timecode and reel name or clip name or fine name etc to identify the clips and apply grades. If you put too clips on the timeline with matching timecode etc, they grade will be applied to both.
It sounds like you don't have discreet timecode/reel name for your clips. You can fix the timecode in the Media pool, or modify the reel name. Or, make a local version of the grade. Details in the manual.
Peter