Brother ribber 850- retaining bar not sitting correctly?

I am having problems with my Brother ribber 850 - my knitting carriage Brothern 950 works fine (I have put in new sponge bar). When Itry to cast on using the ribber I take carriage right to left but not all needles are installing yarn. I have tried all day to solve this without success. When looking at the ribber I noticed (through the slits) that the retainer bar was fixed each end but dropping in the middle. I don't know whatn causes this or how to rectify it. Could this be the problem?