With Fusion, the key to troubleshooting is to remember that it is INCREDIBLY literal to the point of sometimes not being logical.
If I had to guess, your scenario has something like 'Watch Events', 'Read Record', 'Create Record' and 'Update Record'. From there, a couple things could be wrong:
1. When you read your Task record, you're having it pass forward the 'Assigned to ID' field. In that case, it's only going to pass forward the ID associated to the task's Owner, rather than all IDs assigned to the task. (In Reports, this is the difference between filtering for a USER.ID in 'Assigned To ID' and 'Assignments ID'.)
2. If you got past that, and already found the 'Assignments' collection under the Advanced menu when reading the task record, then the resulting array will have multiple IDs if the task had multiple people assigned. Step One troubleshooting - is your 'Update' module getting the inputs you expect it to work with in the first place?
3. If your Update module is getting the complete list, but only assigning one, then you are probably missing a necessary Iterator to tell Fusion to perform your Update for every record in the array.
And if none of this is helpful, and you are able to do so, post a screenshot of the layout of your workflow to see if someone else can spot the challenge.
Katherine