The OEC I generated does not cover the exit date of my FDW even if I encoded the flight date

Kindly take note that OEC is only valid 60 days from date generated and OEC is needed upon exit in the Philippines. It is advisable that this is generated closer to the date of return to Singapore to ensure that validity covers the exit date. You may wait for your OEC to expire and generate a new one upon expiration of the current OEC.