I’m having the same problem, with 720p60 footage (at 59.94 fps) from an HVX200 refusing to convert to 23.98 fps
One common solution to the problem is apparently to import the clips in a new, empty project, and try converting. That hasn’t worked for me, though.
You could also try to Conform your clips using Cinema Tools, but as far as I could tell, clips that won’t convert using the DVCPRO HD Frame Rate Converter within Final Cut Pro will also not conform in Cinema Tools. It says the clip has temporal compression, which from my understanding of the DVCPRO HD codec is simply impossible. Looks like a bug in either QuickTime or Final Cut Pro…
To try the Cinema Tools method, just launch it and select Open Clip from the File menu, then click Conform. If “Conform” is grayed out, chances are you’re experiencing the same bug as I do, and so far I haven’t found a perfect solution to this.
The workaround is to export the clips to the exact same format they are currently in (720p50 DVCPRO HD in your case) using QuickTime Player (should also work with Final Cut Pro, but I haven’t tested), and then try the DVCPRO HD Frame Rate Converter on that clip. The downside, besides the fact that it is time-consuming, is that you lose a little bit of quality because you are actually recompressing the footage a second time. The quality loss might not be visible, but it is there.
If anybody has a better solution, I’d love to hear it.