How old is the TC?
What OS and utility are you using? If you use 6.3 utiity you must turn on ipv6 local link otherwise it won't work in some situations.
See Apple's belated doco.
You can load the older utility into Mountain Lion with a bit of fiddling.. for an older TC it works better.
If you need help with that google for install 5.6 utility mountain lion or ask.