The possible reason is either the mismatch between the expertise listed in the contractor master and the work order, or a contractor is not created / approved. 
While creating a work order, you will come across a field Work Order Scope listing Single Category Scope and Multiple Category Scope. The following are the explanations on the two categories.
Work Order Scope:

1. Single Category Scope: The contractor must match the required skill set. If it does not match, then either:
• Contractor is not created, or
• Contractor is not approved.
2. Multiple Category Scope: No issue is encountered.