Hello there XCnm,
We see that you're unable to adjust your volume past a certain level regardless of the volume slider. We'd like to see how we can help with this.
Are you seeing that adjusting the volume with the buttons allows you to change the volume level, or does it remain the same as if you used the slider?
We'd recommend making sure you're up to date with the latest iOS version on your iPhone to see if that clears up what may be occurring. If you do see an update is available, be sure you have a recent backup of information. If you need help with this, take a look at the following resources:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
If you see that an update isn't related, check out the section labeled "Limit the headphone volume" of this resource:
Adjust the volume on iPhone
We hope this is able to help.
Have a great day!