Itunes synching problems with iPhone 6 plus

I have an iPhone 6 Plus 128GB. Since I've got this new phone (previously had a 5S) I have had constant problems synching with my iTunes library. Both movies and music. I now have to toggle back and forth between 'manually synch' button to get my library (or at least the majority of it) on to my iPhone. Is there a reason why I have to manually synch and not just do a regular synch by selecting the movies and music that I want? Even when manually synching, so for example I have a new movie to add so I just plug in the phone to the computer to try and add that one song, it doesn't work properly and I have to keep toggling the manual synch button (and then clicking synch below) multiple times to get it to work.

Argggh, Apple was supposed to just work! Why all of a sudden these problems that I've never had with any previous iPhones!!!

Help!