It's amazing what a readme will tell you.NEW in version 2.50: The program is now able to detect whether it is necessary to relocate the initial and system programs or not. This improves success rates greatly, especially for European -> American conversions. Most Japanese -> American and Japanese -> European conversions are not affected, unfortunately.
The reason for the different success rates lies in the special design of the Sega CD games' boot blocks. Unfortunately, Sega decided to mix security code and actual game code in the first few sectors, which makes it very difficult (in most cases) to relocate the game code in order to gain space for the lengthy American and European security codes. If a game doesn't use relocatable code (as it is theoretically a Sega programming standard), the conversion might fail.