I haven't had this issue happen often, but its been a few times. When I make French Press coffee my steps are to:
Put coffee grinds in French Press
Pour hot water in
Stir coffee grinds in the water
Place the lid on top
Wait 3 1/2 minutes and then press it down.
Once in a while when I go to press it down, it just doesn't want to press. There is so much pressure resisting underneath. The one time I kept pressing, the coffee and grinds shot out the top and burned me. It seems as if too much air gets trapped underneath but I don't know how. Does anyone know why this happens; if I am doing anything wrong that causes this?