A task needs an owner and a deadline

If you want something to happen, you need to make a single person responsible for it and agree a deadline.

If the activity involves lots of people, you must still make a single individual responsible for making it happen – otherwise everyone just assumes someone else is taking care of it.  You need to assign a cat herder.

If there isn’t a natural deadline then you need to make one up and ensure the assignee knows about it.  You always have some sort of timescale in your head, so it is only fair to let them know what it is.

If you don’t agree a deadline, don’t be surprised when your task keeps getting pushed to the back of the queue.