[Jack-Devel] synchronization with Jack transport

Hello, I have a problem of synchronization with Jack: for a song at a tempo of 120bpm I drive several synths including a hardware Tempest in  slave mode with seq24 disconnected from jack transport and everything is fine. But if I connect seq24 to Jack my Tempest suddenly turns much more slowly, I would say almost half the tempo. Would you have any idea where the problem might come from? thank you in advance for your help, best regards, Philippe