While a tracking number is provided on the receipt by the post office after the package was accepted for shipment, it will not immediately reflect in the postal service' tracking system as their encoding is not done in real-time (this is mainly due to the fact that post offices in the Philippines are largely not computerized).
All packages are brought to the Central Mail Exchange Center (CMEC) where they are further processed for dispatch and where tracking numbers are actually encoded in the system. This is where the "delay" usually happens. It will normally be available after several days. Also, if a package is dispatched towards the end of the week, this will add to the number of days that the tracking number will appear in their system, since post offices are closed during weekends.