I have found that thumb tacks do not do a good job when holding the lining above your head, they work fine to hold the vertical edge seen at the hatch opening. My double sided tape failed there due to the heat, it got all runny.
I found that short self tapping screws 3/8" long did a fine job. My screws were the type that looked like they had a washer under the screw head.
My liner has fallen above the passenger seat as well, I think it is only held up on the edges, time for more screws.