You can't sketch on image from other devices

Testing version:
2.0 test
What were you doing:
I tried to continue draw on my sketch from other devices( draw on Ipad, sketch created by iPhone)
What feature did you use:
sketch
What happened:
It seems like it just viewed sketches from other devices as image, I can’t extend the sketch length
What did you expect to happen:
I hope I can extend the sketch