Problem:

We have an Attribute in a class but its not showing the drop down list of that Attribute.

Solution:

This may occur due to conflicts between similar Attributes pointing to different scopes.

For example: There are 2 attributes with same name A and 2 scopes B and C. A may be pointing to scope B and we may be expecting to see that in Scope C.

Firstly, check if there are 2 attributes with same and same value 

Go to System Administration to check attributes  (example screenshot below):

If there are 2 attributes with the same click the three dots (...) besides the Type of value and check the query

From the database you can find out the Attribute id and see if the id is pointing to the correct scope where should be as expected.