A QR code won't open.

Scan the code with your own phone first and note what the screen says. The message tells you what's wrong.

Steps

Scan the code and read the message on your phone.
In the sidebar, under Content, click Exhibits & Objects.
If the message was about the exhibit being closed, set the exhibit's Status to Active. It saves straight away.

“This exhibit is having a rest”: the exhibit's Status is Hidden. Set it to Active when it reopens.
“Under maintenance”: the Status is Maintenance. Set it to Active when it reopens.
“We can't find that exhibit”: the exhibit has been deleted, or the sign is an old code. Download a new code for the exhibit, or take the sign down if it has closed for good.
“We can't find that one”: an animal's own code, where the animal has been deleted or moved to another exhibit. Download its code again from its new exhibit.
“Something went adrift”: the phone couldn't load the guide. Check there's phone signal or Wi-Fi at the sign, then try again.

Move closer, hold the phone steady and check there's no glare on the code.
Check the white border round the code hasn't been trimmed off or covered.
Make sure the code isn't creased, scratched or faded. Reprint it from the SVG file if it is.

Renaming an exhibit doesn't break its code. The code is built from the exhibit's ID.