desktop itunes 'add to device' option stopped showing. Preventing from downloading music into iphone.

I have had old songs from other computers and old emails as well as self made songs /audios and videos transferred to my current computer. They have been downloaded into my desktop files. I've always made the same steps which are: after downloading to desktop files, I right click and it gives me the option to 'open in itunes'. From there, the song is downloaded into my desktop itunes but It still needs to download into my iphone, so I right click the song, and it gives me the option to 'add to device'. It shows the little phone icon on the side which I click and it automatically downloads into my iPhone.

Recently, the 'add to device' stopped showing and some self made songs / videos were removed from my iphone's itunes yet are still available on my desktop itunes.

It has enough storage. I deleted and reinstalled the app, updated both desktop and iphone, but nothing seems to work. infact, it seems my desktop itunes hasn't been responding well and is getting slower to open as well as the main issue.

I don't know if it could be related, but a few days before this happened I had decided to get a 3 month free itunes subscription. I canceled a few days later when I noticed I couldn't download some new songs from my desktop into itunes.

I would really appreciate any help. Thank you.