Sometimes my tire rubs on the floor mat when I'm climbing out of the saddle on my Rock And Roll trainer. Can I reduce the sag?
Be sure your Rock And Roll is on firm flooring, carpet can worsen sag.
Finally, be sure your bike tire is centered on the roller by adjusting the left or right cone cups accordingly, this will help any leaning issues as well.
take care, this happened to me the weld on the lower unit had broken and the whole unit folded and back wheel hit floor.
A crack starting could be the cause of any "sag"