itunes not synching with iphone to transfer videos

Got used to transferring music videos from itunes to my iphone 6 plus as it would normally do it automatically when i plugged the phone into the computer.Then that stopped and i had to go File>Devices>Sync Iphone to get all the recent videos transferred.Now neither methods work and so I cant transfer new video to my iphone.I have the latest version of IOS but it still doesnt work