So difficult to get answers

Because you have to post what the problem is before anyone can help you.

Great! You have discovered one of the universal truths of the universe: When in doubt, reboot 😏

More seriously, the reason this frequently works is that Bluetooth is a very complex communications method based on frequency hopping, or what is called “spread spectrum” protocols. The technique was invented and patented by the actress Hedy Lamar (who was also an engineer), originally for sending undecipherable messages in wartime. The two communicating ends change their frequency of transmission every few milliseconds. The communication works because the two ends have previously “agreed” on the frequency switch pattern, so the sender and receiver are always on the same frequency, even though it keeps changing. Except when they get out of sync or “collide” with another “user". So when you “forget” and restart Bluetooth, the two ends negotiate a new pattern that will usually work better.

A side advantage of spread spectrum is that there can be many communication streams using the same frequency band. As they are all changing frequency at what appears to be a random pattern, the probability of two of the using the same exact frequency at the exact same time is small, but as it isn’t zero, when this happens communication breaks down, and “rebooting” the protocol is a good fix. The ability to use the same band for multiple conversations is Bluetooth (and Wi-Fi) use it.

Well, as users no one in this forum can answer a “why” question. There are a number of hearing aids that work with iPhones, but ultimately it is up to the hearing aid manufacturers to implement their side of the communications.

Here is a discussion of the features that iPhones support for hearing impairments→Hearing accessibility features in iOS - Apple Support

TRy this link...

Use hearing devices with iPhone - Apple Support

Do your hearing aids have an App? Mine do, though sometimes the app fails to keep up with Apple's updates.