If you filed the 15% first it would have asked you if you want to lock the data up to the VAT filing. I assume you said yes and provided a password. That can be the only explanation I can come up with.

To reset or change the date to which transactions are locked, goto Edit, Preferences, Accounting and then change the date and password at the bottom of the screen.

Then, I suggest display the VAT report again. If it is blank then goto the vat journal in the vendor account. Delete it (you can't edit or void it. Then display and file again.