Take the central nut off. Try and remove the planet hub by wiggling it with a downwards motion.
Check that the gear (cog) on top of the beater socket is not damaged (sometimes they’re made of plastic and have broken or split). If necessary fit a new gear (available online) and secure with a 5 mm starlock washer/push-on fastener. Whilst its off you could clean off any old dirt/grease using a cloth with some white spirit, and re-lubricate around the teeth on the underside of the gearbox with a smear of grease. Also check that the hole in the middle of the hub is still “D” shaped - these can wear down with age.
Replace the planet hub and refit the nut using a new washer (plastic or fibre) between the planet hub and the nut. An M6 nylon washer should be the right size. Do the nut up with a spanner, but do not overtighten as you don’t want to strip the threads. Note that there may be a small gap between the hub and the nut if the new washer is thicker than the original. This will not affect operation.