Casting off with a Brother KR-89?

Hi there, I have a G-carriage which does ribbing etc. properly, however, when I want to cast off it skips the first 6 stitches and then carries on as it should.
Have tried on different needles, replaced sponge bar, replaced the G-needle etc. but it refuses to cast off properly. Would appreciate some help! Thanks so much

needlebed
September 2016