Pro 1 clock thru


Recently got a pro1 and all works fine with the exception that if I am using with an external sequencer and passing the clock signal from said sequencer through the pro 1 to my zoom rt123 drum machine the drum machine will try to start and display the first beat but will not get any further. 
if I use a cable splitting the signal from the out of my sequencer and take one output to the pro1 and the other to the drum machine everything works as expected. 
so I know some signal is passing through the pro1 but it's not rubbing the drum machine as it's supposed to. 
any ideas greatly appreciated