Lexmark P4350 not printing on all of the page?

Hi

I have a Lexmark P4350 and when it prints on the paper using the "borderless" option, it leaves a small strip down the edge of the page not printed on (about 1/4 inch). It used to print right up to the edge, but it doesn't any more (so I know it's wrong - it's not me not feeding the paper in properly or anything). I have heard something about the printer sensor not recognising where the end of the paper actually is. Does anyone know how to fix this, please, as it's really irritating when bits are being missed off of the paper.

Thanks very much