seed heads on stags horn (rhus)

Hi am new on here but wondering if any one can help? i have 3 rhus in the garden 2 are "tiger eyes" the other rhus glabra. Anyway the glabra and one of the others are getting the attractive seed heads however they get to a certain size then dry up and drop off before they get to full size giving their show in autumn. do they need pollinating or male and female plants etc. Am becoming seriously frustrated as that's why i grew them as there's some cracking specimens in Durham near where i live which look stunning in the autumn any ideas appreciated