can't copy and paste in fb.

SOMETIMES but not always, when I put my finger on a post in FB, the little copy bubble will pop up for like one second and disappear before I can touch it. Mostly though nothing happens when I hold ny finger down on anything in fb. I can use copy/paste on other apps on my iphone 8, but not fb. I see alot of people have posted about this but no one seems to have an answer. I had the same problem on my iphone 6. So is this a fb problem or an apple problem or do I have to change something in my settings either on my phone or on fb? Can someone please give me an answer?