Hello pepijn32,
Welcome to the Apple Support Communities!
Make sure your Dolby Atmos settings are correctly set for your iPhone. Here's a guide to help with reviewing those settings: About Spatial Audio with Dolby Atmos in Apple Music - Apple Support.
Choose how to listen to Dolby Atmos on your iPhone or iPad
1. Update your iPhone or iPad to the latest version of iOS or iPadOS.
2. Go to Settings.
3. Tap Music.
4. Under Audio, tap Dolby Atmos.
5. Choose Automatic, Always On, or Off.
As it shows there, if your settings show Automatic, you need the following for it to work:
Supported songs automatically play in Dolby Atmos when you're listening with:
- AirPods Pro, AirPods Max, AirPods (3rd generation), or Beats Fit Pro with Spatial Audio turned on
- AirPods (1st or 2nd generation)
- BeatsX, Beats Solo3 Wireless, Beats Studio3, Powerbeats3 Wireless, Beats Flex, Powerbeats Pro, Beats Solo Pro, or Beats Studio Buds
- Any Bluetooth headphones that you categorize as headphones on your iPhone or iPad
- The built-in speakers on an iPhone XS or later (except iPhone SE), iPad Pro 12.9-inch (3rd generation or later), iPad Pro 11-inch, or iPad Air (4th generation or later)
Or if Always On is enabled, make sure you're plugging headphone directly into your iPhone in order for it to work.
There's also a great FAQ section at that bottom of that page that has some great information about Spatial Audio with Dolby Atmos, so be sure to look that over as well.
Best wishes!