Theres a few different issues it might be. Are you using all the focus points or just one? The camera actually needs something to focus on, sometimes just moving the focus point over an edge of say a table (whatever) will give it something to focus on, blank areas with low contrast are famously difficult to get with auto focus. Some cameras have a focus assist light, I don't know if your model has one, if not sometimes a small torch will give it enough light to focus, or use the one on your flash if you have one.