If you have other light present it can easily "wash out" low level luminescence.
You should have NO other light to start.
Focus with a light on. Either use manual focus or, if your camera allows, set to manual focus, toggle temporarily into AF, focus and then drop back to MF so the camera is focused.
Then turn other lighting off so that the luciferase is the only lighting source, expose from there and see what you get.
If that does not work, manual exposure as well as manual focusing allows you to control what the camera does and to expose across a range of likely exposures.
I just tried photographing my not especially bright luminous wrist watch dial in total darkness. At 250 mm and watch about 500 mm away so the watch filled 1/3 of width of frame exposure was 6 seconds at f5.6 at 800 ISO. Individual hour markers, which are not very bright to the eye in roomlight, glow brightly in the photo when they are the sole light source.
Thusly. Focusing "could have been vastly better [tm] :-)
6S, f5.6, ISO800.
Key questions are - are you using other lighting and, what ARE you seeing in your images.
These many Luciferase images suggest a reasonable light level and do not look like they should be hard to photograph.