Projects with big organizations tend to involve a lot of people on the project side. Whether it’s a whole team you have to go through or just a handful of people, this can create too much drag on the project. Instead of having one or two people sign off on a mock-up, you have six different people providing six different points of view with no consensus in sight.
The best way to manage this situation is to establish a lead point of contact. Who is the chief representative from the organization? Usually, this is the person who ultimately signs off on everything. You can reduce the amount of friction in the project by going directly to them. It should then be that person’s option to seek input from their directs or peers.
You may even want to state outright from the beginning of the project that you need a primary point of contact to work with. If it’s not easily identifiable who the chief representative is, this may be the route to take.