Loose Gate Posts

[Catmandave]

I had a new perimeter wall complete with gate posts built just over a year ago. I have recently had new railings and gates fitted. I have now discovered that the gate posts for the side gates can both be moved by hand. My question is "How do I remedy this, am I ok just to cut out the mortar at the weak point and re-mortar the joint and if so should I include a bonding agent i.e PVA in the mix?" The original builder is not interested in remedying the fault even though he charged me well over £4,000.00. Any help or suggestions would be appreciated.