You don't see any output from the Tag Debug block because it only shows the tags, not the data. You actually don't have any tags in your data. You just have a PMT object with a NIL CAR and a populated CDR. Since there are no elements in the CAR, you get no tags. I personally got confused because I thought that if I passed in a valid CAR/CDR combo to the pmt generator then it would just overwrite the CDR portion. Not so, it actually ignores whatever data comes in. Check out
http://gnuradio.org/doc/doxygen/page_pmt.html for more info about PMT objects.