It's been a long time since I owned a phone with a microSD slot, but I'd guess either check settings or permissions of the gallery app to make sure it has SD access. That should only be an issue for writing to SD, but as the gallery app can modify or delete photos that might be necessary.
As I say, only guessing: I don't have a device running an even moderately recent version of Android that also has a microSD slot, so have no experience of this since SD access began to be restricted.