Check marked titles are not feasible to synch.Mac to Iphone6

Dear apple community,

hopefully one of you can help me:

I would like to synchronize my selected titles at my iTunes on my Mac to my iPhone 6. With my last iPhones this wasn't an issue, but when I synchronize now, it keeps songs that aren't selected and doesn't add songs I activated within my iTunes on my Mac with a check mark.

I tried different recommendations of the internet, saying to activate the "manual managing" of the titles, but I do not want to drag and drop every single title, especially I already marked them in the iTunes/Mac list.

Thank you a lot in advance and best wishes,

Teddytree

PS: If you also got a hint, how I can copy the videos of my iPhone to my Mac this would be great!!!