Kenwood Chef A701?

Beaters seem to slip down slightly from planet hub when mixing effectively rendering mixing action useless. Could the problem be with the small gear in the planet hub or in the part holding the shaft ... bayonet ... ? Have opened the gear mechanism and all seems to be in order. Have also changed the drive belt, although not necessary. Needless to say, if this problem can be fixed easily, should like the beater to produce a few more cakes and loaves in the next 30 years! Thanks for all help.