Unreasonable expectations are one of the specific ways that communication is a problem is around delivery times. Requesters can have unrealistic expectations and make their program plans around them. Purchasing staff tend not to feel they can correct the requester. It is usually up to the team lead to make sure non-purchasing staff (those making requests) understand that the purchasing process can take time and that they must plan for this.
Here’s the message you can give to staff to help things along:
Requesters MUST speak with purchasing as they are making their program plans, not after those plans have been finalized. Meanwhile, Purchasing Staff must tell requesters, and work with them to find a reasonable alternative, if the delivery times they request are unrealistic.
Also note that Purchasing Staff, especially during an emergency response, may not know how long something will take. This is one reason that communication is so important. We will all be learning as we go.