aha, Well I've been doing it totally wrongly all this time but it's worked up until now.
The way I've been doing it is to click on embed in youtube which gives you a long blue address in the box to the left of the embed button. I've been copying that and pasting it into a reply box.
Then surrounding it with the youtube tags.
As I say that's been working for me for months but I suddenly thought of the page address bar
Hadn't realized people were referring to that when giving instructions
Still doesn't explain why my old method has stopped working though.