I found this on the internet Sheila

'Robinia trees do not require regular pruning. To tidy up your pink false acacia tree, you can remove any crowded or damaged branches in mid-summer, this will give enough time for the wounds to heal before the frost arrives. Make sure to wear protective gloves and long sleeved clothing when pruning robinia trees.'

perhaps it is just too young yet to be 'bushier'.

Thanks very much Eileen. I expect you’re right, and I just need to be more patient.