The difference in Chicago is that True Oldies 94.7 is totally different than what WCBS is doing in NYC. Yes, it is technically "Oldies" on WCBS, but they are playing music mostly from the 70's, with some 60's and 80's mixed in. 94.7/Chicago is playing mostly 60's, with some 50's and 70's mixed in, so it's technically two different demographics, so WJMK could flip to a format similar to WCBS and possibly do better than they are doing right now.
Totally different in DFW, where CBS already owns the "Oldies/Classic Hits (whatever they're calling it at the moment)" station. CBS, as we all know, has multiple underperforming formats in the market. Jack is just the one that stands out the most. I've never understood why this frequency has had so many problems trying to find an audience. It hasn't been successful since 100.3 JAMZ.