Why is locked screen notifications not working?

[Caboz]

Hello guys!

I have an issue with locked screen notifications.
When my screen is unlocked i receive notifications with no issues but as soon as the screen is locked i don't receive any notifications or even hear a sound if i received any message from any app, for example WhatsApp is working fine when screen is unlocked but when it's locked i don't get any notifications.

Android system: Lollipop 5.1
Phone model: Philips Xenium V787

I've tried the following solutions but none of them worked out:
- Settings> Sound & notification> Notification> When device is locked> Checked "Show all notification content".
- Checked app notifications in phone settings.
- Advanced Wi-Fi> Keep Wi-Fi on during sleep> Checked "Always"
- Reset WhatsApp notifications.
- Reinstalling WhatsApp.
- Cleared partition cache from recovery mode.

What I've noticed is that Gmail notifications work fine, even when screen is locked i still hear a notifications sound whenever i receive an email but for the rest of the apps, i don't receive any notifications at all unless the screen is unlocked or the app is opened.

Would appreciate your suggestions to solve this issue.

Thanks in advance.