Why does my lochinvar boiler keep shutting down ??

For flame failure, if you indeed have a proper flame when the unit fires, you will need to check the flame proving components; possible, air switch, flame sensor rod, electrical connections, sensors, blowback switch or rollout switch. If you do not have an understanding of the components involved or the operation of the flame sensing system, to avoid personal injury and possible damage I would advise you to have it checked by a qualified HVAC service company.