Sometimes the call recording requires root and kernel support. This might be available in certain versions or in certain Android versions. For example, a new update for your phone might cause issues with recording. I know I experienced this issue when a feature in the Android Kernel was disabled after I updated the phone, and the call recording wasn't working anymore.
It's best to report this issue to the developer, because this could also be a bug in the application's code.
- Contact info: firstname.lastname@example.org