It is usually the height adjuster arm, they are made of spring steel and either start to crack or just bend and don't hold the pressure against the rack as well. it is worth pulling it off and seeing if it just needs a straighten to hold it better
I fixed my neighbours, same problem. A piece of wire 60cm ,wrap around handle , set at height you want, and tie it off at opposite handle.... it works. speedy
........................Keep your blades sharp......................
