thread trim

I use a machine by Barudan, model BEVY-Z1504C SN: 4106362L07. One of the heads is not trimming properly on any of the colors. I replaced both blades, thinking that that would help, but it did not. I also have removed the front plate and cleaned underneath, and adjusted the arm from the back of the machine that extends under the front plate. It is still not trimming. What can I do to fix this problem? Is it a timing issue?

Location: 
United States