Volume buttons stop working in some apps frequently

Begin with a forced-restart of your iPad - as this will often clear an isolated software issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

If the problem persists after performing the restart, it may be necessary to reset your iPad settings. While this reset will not cause any loss of data, as a precaution, it is always wise to ensure that you have a current iCloud or iTunes backup before performing the reset:

How to back up your iPhone, iPad, and iPod touch - Apple Support

When you are happy to proceed, you can perform the reset from iPad settings:

Settings > General > Transfer or Reset iPad > Reset > Reset All Settings

If the problem still doesn’t clear, the next step is to reset the iPad to factory settings - after which you will need to restore your data from your backup:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Otherwise, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

Performing a regular shutdown will preserve system and process state data (including some non-volatile caches). This state data is rapidly reloaded upon subsequent restart of the iPad. Reloading a previously saved state is both faster and more power-efficient than reinitialising the OS and running Apps. However, while reload of saved state facilitates quick startup, restoring state data may also restore a faulting process.

By contrast, a forced-restart does not preserve/save system state, but instead will for force both system and App processes to be reinitialised cleanly at startup - and in the process will clear faulting process data from the system.

In understanding the fundamental difference between a regular shut-down/restart - versus a forced restart - you will hopefully now understand when the latter is an appropriate step. While a shut-down and restart is both perfectly valid and recommended when your iPad is running normally without issue, a forced-restart is appropriate when attempting to clear a software issue.

While a forced-restart cannot resolve a hardware issue, it is sometimes the only method by which to cleanly restart an iPad to a running state without reloading a fault condition. Reinitialising without saved state or cached data may be the only manner by which a problem can be resolved. In extremis, it may even be necessary to perform a factory restore - which will both erase the device and load a fresh copy of the most recent version of iPadOS supported by the iPad.