Ok, what version of APRSIS32 are you running?  If you freshly downloaded it and haven't yet set it for development mode, you'll want to do that.

http://aprsisce.wikidot.com/doc:development-version

Once running the latest development version, go to View / Logs / Port(<YourPortName>) and enable it.  Then disable the RF port and enable it again and see what the trace log shows as packets are received.

I don't think you mentioned what port type you're using in APRSIS32?  If the TNC is locked in KISS mode, then it should be Simply(KISS).  The KISS port type is for Kenwood APRS radios prior to the D74.