I had the same problem with my Brother ps1500! Every time I used the thread cutter, it would unthread. I was beyond frustrated. I found the solution! I was threading the needle wrong! I threading right to left like the manual showed but after looking closely, it has to be threaded left to right. And another thing, the way the thread is coming off the bobbin makes a difference! Mine has to be coming off counterclock wise if I turn the bobbin around the cutter will unthread the needle. That solved my problem. It could be worth a try for you.