In my personal experience, I have come across many situations when a consultant gets stuck in any configuration job. For example, he gets stuck with the relationship among plant, company code, vendor, purchase organization and material and thus he is not able to create a purchase order for testing this relationship among these entities. Sometimes, he has created a material but it is not working as per his requirement (no automatic assignment possible for this material etc.). Even after referring documentation and googling to find out answers to his problem does not help. So what he is gonna do?
It happens to many people. The first thing to do in this situation is to take a break and have a walk or tea or get some fresh air. Next, he should concentrate on the basics. He should think how he had created the relationships among entities in SAP. He should check them in IMG. Then he should make a mental picture or write down the relationship diagram and ensure that he is not picking a wrong entity (many times we pick a plant 1000 even though we had defined the relationship of the purchase org. with a plant 3000). this is the most common mistake people make.
For example i have this scenario:
I have a material MM01 which i have defined for plant PP01. I have defined a vendor VV01 for purchase org. PO01. When I create a PO, I find that the material MM01 can not be found in plant PP01. When I investigate I found that it was actually plant PP02 and not PP01. Similarly, sometimes on the PO the vendor VV01 could not be selected as system saying that vendor VV01 does not exist for purchase org. PO01. Later you find that you actually defined vendor VV02 for purchase org PO01 and not VV01.
So when you define your organization structure next time, make sure that you make the relationship among entities very clearly and also have a written reference for the same.