To reproduce the behaviour 
1) open Thunar and go to a directory
2) if "detailed list" is not set, activate it (Ctrl-2) 
3) then open a sub-directory (assuming you have one)
4) try to select a file or any other directory inside of it. Result: not possible

Additionally if you hit Ctrl-1 (View as Icons) and then Ctrl-2; then you can select the files.

It only happens with "View as detailed list"