Why Brother HK 840 punchcard moves three rows at once?

When moving the carriage either right or left the punchcard rotates three rows. We've tightened the belt, but didn't make a difference. The little orange plastic knob under the arrows that indicate which direction the carriage went is not seated where it looks as though it should be, on the taller of the two metal protrusions from the right side belt gear. The gear rotates three times with only one carriage pass. Please help.