Unable to update period indicator

Hello Experts,

Here is my scenario :

One of my customer is trying to create a service call with previous financial year..

But series is not getting proper reflected with the posted date (It should get reflect in previous finance year)..

I have checked all the root cause

1. Document Numbering is proper or not.

2. Posting Periods is proper or not.

What could other root cause..

Please suggest as it is a bit urgent !!