My Arc app is set for Daily calendar. For some dates when I export Timeline as image; I get a blank image. For some other dates the images are fine. What can be the reason?
Yep. Super annoying eh.
This is because of some bug/deficiency in iOS’s “create an image from this view” feature. If the view is too “big” then sometimes it silently fails, and just returns an empty image file (albeit with the correct pixel dimensions), without actually reporting any error.
Because there’s no error reported, Arc doesn’t know anything went wrong, and unhelpfully still offers up the file for you to save, blissfully unaware that it’s useless 
By “too big”, it seems to be not just the timeline being long, as in a lot of timeline items, but also things like if there’s a lot of photo thumbnails in the day too. So a short-ish day but with lots of photo thumbnails for every time will sometimes also trigger the bug / failure.
I’ve had it on my todos to build a custom “create an image from this view” feature, to replace use of the built in iOS one. But it’ll be a bunch of tedious / fiddly / time consuming work, so I just never find time to get around to it. But … someday I will! It annoys me as much as everyone else!