Problem with drag and drop of music files

I think you need to investigate further then, because usually, one uses iTunes itself to create a digital copy of CD tracks (rather than drag-and-dropping them into your Library) and they have never been an issue. I'm going to guess that it's the MP3s from YouTube that are not being added. You need to identify your original source of any songs that will not successfully add to your Library, not simply include every source. Let's look at MP3s from YouTube first, I'll come back to the CD tracks later.

Regarding music taken from YouTube: presumably you used an MP3 convertor programme, designed specifically to create MP3s from YouTube. Something has changed recently. No one knows for sure who changed things (probably YouTube), but it now means that it is no longer possible to add MP3s copied from YouTube to your iTunes Library. This will be to prevent copyright theft.

As far as the tracks from CDs are concerned, as stated, the usual method is to use iTunes to "rip" (copy) from the CD, direct into your iTunes Library. Do you actually have any tracks that would not copy from CD, into your Library?

Lastly, if you used another programme to create a digital copy of a CD and then tried dragging that digital copy into your Library, it should work, unless you used the same programme as the one used for copying from YouTube.

So which songs are affected?

Where did you obtain the song? Is it a purchased from an online retailer such as Amazon, iTunes or was it copied from YouTube?