Re-fixing parquet floor tiles

Hi!
A couple of rooms in my house have parquet wood floor tiles. The house was built in about 1980. A few of the tiles have lifted and become loose. I can quite easily remove them intact, but the back of the tiles and floor is covered in sticky black tar.
From a bit of research, I guess this was tar paper, used to fix the tiles. Out of interested, what would normally cause it to come loose like that?
I found quite a lot of articles online about removing this stuff when you want to have a clear floor surface, but what I want to is to stick the tiles back down. Should I try heating the tar - perhaps with a heat gun - and replacing the tiles? Or should I try a new adhesive? If so what, and will I need to remove the tar before using it?
Thanks!
John
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Its bitumen. Remelting it is the easy option, but there might not be enough in the right place to work. If it fails, add some more bitumen. Either solid bitumen you melt, or liquid solvent in bitumen, whch then takes an age to set. No need to remove old bit, unless its in the way. If you do end up getting any on the top surface, white spirit removes it, but it also tends to cause more to ooze up.
NT
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