I have to say that I've never had a problem in getting corrections on GPS datacards. My impression is that Jeppesen take the fault seriously, as well they should!
If you're not getting anywhere with Jepps kriskross, I'd have to say that the error might be in the equipment you use. I find that Jepps always check their own data, upon notification and, if they can't find any error, they'll query the aviation regulator who provided the data in the first place. I'd say this is what has happened in your case and that the regulator has said that the data is correct.
Just a thought, anyway...