Pencil line won't merge all the time


I have a drawing made of pencil line and a color. After i save the scene and reopen it and try to add another pencil line and merge it to the old ones it doesn’t work. However if i draw several new pencil lines in the same drawing merging is working fine. The problem is to merge the old pencil lines with the new ones. I am not sure why is this happening and how to solve it.

Any advice?


Here is the update:

I have changed the line color from black to green or any other color and reverted back to black. Merging started to work as usual.