When players get completely stuck, they can simply use the round to feed more raw material into the game.
For example, a stuck player could spend a round just introducing new elements, or developing existing elements, or throwing out combination ideas.
By doing this, players stimulate their own imagination, possibly finding ideas they can use in subsequent rounds, as well as feeding ideas to other players who read their replies.
Tip: Lower the stakes
Reinforce that players don't have to write a great story reply every time, especially when there are other players sharing the load. It's fine to feed the team!
However, there's a difference between genuinely brainstorming and throwing out ideas and attempting to derail the game. If you are approving replies, you'll need to judge if players are genuinely feeding the team or if they are being deliberately disruptive.