Ok, first thing first, where are you thermometers? Make sure that you have a very accurate reading of not only the cage itself, but the tape too. If the tape is not heating up well (which has happened to me) than recheck the connections. I had a problem with my solders coming loose because i used a silver/lead solder. I had to readjust with something that melted at a higher temperature and then i covered it with Aluminum Tape AND electrical tape.
If it's not the tape or the probes it may be the bedding. I would reccoment taking an empty sterilite container and trying it out on the remaining few feet of Flexwatt without anything in it. If it heats up well than it's the bedding. If not you may want to concider trying another heating method temporarily.