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.

 

I told you before , a small straight pin weaved in the liner will hold it , till you get it recovered. No more wondering , "What the he//s touching my head"...

 

These screws are not going to move easily. If I added more in a square pattern I could achieve a quilted look to the head lining.

 

Be careful with a liner that is loose. When I was kid I had one rip out with windows down going 70mph. All the dried out foam insulation got in my eyes and I almost crashed.

 

That is an excellent point that we all should be aware of.