Checked back with Tweakie regarding this and no explanation available response was

"I have not looked into the reasons behind opto - opto problems, perhaps it's timing related, it is just that this issue has been reported on the forum many times in the past. Tweakie."

so no nearer to knowing the problem, unfortunately the Bob I used I was given new and sealed so no real information on it, but out of interest I connected it on the Seig X1 conversion which does not have opto isolated drivers and it worked perfectly. It may just be the Bob is a poor quality cheap import possible others reported are same, more may just have been replaced as faulty, it will remain a mystery for now.