This may apply, make sure you have all of the gelatine in the bowl! If you only dump in half of the package (by mistake), the jello won't set -- because there's too much water for the amount of gelatine. If you try to add the rest of it later, when the water is no longer boiling hot, it won't dissolve... and it (1) won't set *and* (2) will have bits of grainy texture to it. Also try to use a glass container, it sets better in them.