Music keeps disappearing

this Is really stressing me out..

I download music myself on to my iPhone so I don’t use Apple Music... every time I put new songs on.. they appear on my iPhone... an hour left a bunch of them just disappear? And when I go on to iTunes and connect my iPhone and search for the missing song on iTunes on the iPhone, it comes up with an explanation mark.. why does this keep happening... it’s been occurring for a month or two now...

I’m using IOS 13.6 on iPhone pro max

I have an iPhone 6s and I don’t have the problem on there..