These trees often suffer from disease and insect attack. You appear to have ok soil drainage. Assume you have good sun exposure? The existing trees look to be too close to the Leyland
(or any other tree for that matter). I would recommend 12' minimum clearance to other trees. Leyland does not have good drought resistance here in central Texas with 32" annual rainfall. Here it should be watered during periods of hot dry weather and it may still fail. Trim off the dead parts and throw the dead limbs away.