I would, in order:
1) Clean Accessory contacts with rubber eraser or a light alcohol cleaner. Do not use metal on metal: this will rapidly lead to having to replace the accessory connector.
2) Don't clamp the cable down really hard. XPRs are finicky with accessory cable tightness, so just turn the radio on, put the cable barely on the screw and gently tighten it bit by bit until your computer goes LO-HI to say it's connected.
3) Try a different cable, OEM in particular if you haven't already. Also, don't use the PMKN4012-A suffix cable if you can get a -B cable, the -A can cause problems.
and,
////// IF YOU ARE INTERESTED IN KEEPING THE INTRINSIC SAFETY RATING FOR CONFORMITY IN USE OR RESALE, DO NOT DO THE FOLLOWING //////
If you're NOT interested in keeping the IS rating and intend to be the sole owner of the radio for the rest of its existence (seriously, think twice before voiding IS if you want to resell it) AND you have some experience with careful electronics, you can pull the guts and put them in the 6500 shell to see if it will read through that accessory connector (which would tell you your 6550IS housing's connector is bad).
But seriously, I do mean the warnings above. I yank radio guts and do swaps like that, but I'm a service tech and I have a static-safe bench and so forth to do it safely for the radio. If you're going to do it, take care.