Tag axles and sliding 5th wheel

Was recently driving a tag axle and couldn’t seem to run with the tag up whilst I had a trailer hooked up. Was the same when I was running empty with a 10m trailer which makes me think I was doing something wrong. When I tried to lift the axle, I got a warning message on the dash and nothing happens. Could it be that the 5th wheel is sitting too far back?

Which brings me onto how to use the sliding 5th wheel - not got a clue! When should I be using it and how would I do it?

Was it a Renault t range by any chance??

No, a scania r560