A lot of riders tend to ‘freeze’, especially with the hands, in lateral work, and ‘stop riding’, probably because they are concentrating so much on angle, bend, rhythm, forwardness, which seat bone to weight etc… there’s a lot to monitor!
One solution is to try doing all the movements in walk first. As the horse holds his head stiller in trot than in walk, it is easier to lose the elastic nature of the rein contact, so practising in walk first means that you’ll keep the softness of the elbows and the following hands. Focussing on allowing the head and neck to move means the rider is free to ride the movement, staying elastic and responsive. Obviously make sure the walk is kept flowing forward with purpose.