That's because our business is like that. Most cases are usually completed in a few days, but others may take weeks, even months. Even years. So, we may return to a case and geographical location more than once as sources become available and questions present themselves based on earlier findings.
The clean cases are easily resolved. It's the dirty cases where things get complicated because we handle assignments with a view towards possible criminal charges or litigation. That means gathering evidence: proof, readying witnesses and making them available.
It's not enough to think or even know about a wrong-doing; it must be proven.