I can not see the imported layers in onion skin

I paint classic animation by hand then scan the images and import them into the program as a sequence of black and white line art. The problem is that when I turn on onion skin I can’t see the onion layers.
Imported images are represented as blue rectangles in the timeline, and ordinary drawings as white.When I’m in a white frame I can see the image in the blue frame but only one frame before.If you understend what I mean please help.

Hi, There,
Please send your screenshots to support@toonboom.com
Without see the actual drawings, we could not tell what caused the problem.