only one hearing aid is pairing

This has happened to me in the past and AFAIK there are two causes:

1. Nothing actually wrong but it sometimes takes me a few attempts to get successful pairing. Sometimes only one will pair. Fresh battery often makes it work.

1. Once, my audiologist had to send one of my aids back to ReSound. They sent a brand new replacement, loaded with my settings. I picked it up and could only pair one HA. I could pair either the left or the right but not both at the same time. The problem was that the replacement had not been paired with original so the phone saw them as separate devices not a pair. This has to be done by your audiologist. You can test if this is the problem by putting both aids in.....if they are correctly paired, pressing a prog or volume button on one aid will change the other. They talk to each other if paired.

Do you have your HAs connected to another iDevice (i.e. iPad)?

I have this issue when I am near both my iPhone and iPad (both are connected to the HAs), the handoff does not work for me and I experience audio going to one HA rather than the two.

The fix for me is to turn the bluetooth to off for the iDevice I am not using.