W
White Room
Same as me , I use doorstops on top of bankers around the edge if i do a S&C screed - which hasn't been for a while - .Without the doorstop , I think if you tamp your bankers in a bit more , making them a bit more solid and try not to lean on the straight edge , also try taking a bit less off each pull.
Diggy
I never tamp the screed, just bring the screed about 10mm higher and push forward with an ali screed rule with a small amount of body weight on the screed rule.