My master bath never gets really cool like the rest of the house. I had a new A/C Heat Pump installed last year (not for this - the old was worn out and we added an addition th our den). This bathroom has never, even before the addition/new unit, cooled off properly.
It is supplied by a branch duct coming off the same large duct that feeds the adjoining bedroom (which is always comfortable).
Any ideas?

Wrong size duct to bathroom or improper take off for duct. This is a common mistake in the HVAC trade.