Can’t paste (cmd+v) into some text fields, but can in others

As the title says, I can't paste (cmd+v) into some text fields, but can in others.

Its not limited to particular apps. In Mail I can paste into the main text of the body, but if I want to paste a link into the add link box then I have to 'right click' and paste the text.

Any ideas?

Thanks in advance.