I think the excess clearance might be a good start. If you think about the way the pilot clutch works through the two toggle arms, then the more the clearance the more the two toggles are nearly perpendicular and the more force they can impart as the linings are starting to touch the inside of the drum. If the toggles are still at a shallow angle, then I would be looking at weak centralising springs. I also like to use a needle roller thrust race (my design) in the centre of the pilot outer plate to ensure that it lifts square.