Hi there Rehankw,
Thanks for joining us in Apple Support Communities. We understand the volume is lower than expected on your iPhone for notifications. We're glad you're here, and we'd like to learn more to see how we can help.
To start, which iOS version is listed in Settings > General > About? Are you only noticing the volume is too low for notifications in Messages or WhatsApp, or is this happening with notifications from other apps too? If you change the tone for notifications, does that make a difference? Learn more here:
Use notifications on your iPhone, iPad, and iPod touch
"When you’re on the phone or listening to songs, movies, or other media, the buttons on the side of iPhone adjust the audio volume. Otherwise, the buttons control the volume for the ringer, alerts, and other sound effects."
Keeping that in mind, let's be sure you've checked your volume settings with the steps here:
Adjust the volume on iPhone
Send over these details, and we'll take a closer look from there.
Cheers.