Zend certified PHP/Magento developer

Magento 1.9.0.1 sales_flat_invoice_grid missing rows

We are missing invoices in backend’s invoice grid. I took a look to the database and it seems that the invoices are not in sales_flat_invoice_grid table. The invoices do appear in sales_flat_invoice table.

What are the reasons that could cause the invoices not being populated to the sales_flat_invoice_grid table?