Speech blocks makes me look dumb

Speech blocks makes me look dumb Whenever I’m in class and I get asked a question and i get hit with a speech block I just act like I don’t know the answer to the question when I really do. I’ll attempt to speak for like 1-2 seconds and when I realise the speech block won’t go I just answer with “I don’t know”. WHY IS IT I CAN SAY “I DONT KNOW” BUT I CANT ANSWER A FREAKING QUESTION