The joists are likely on either 16 inch or 24 inch on center (OC) spacing, though rarely some are 19.2 inch oc. They will usually span in the direction of the shortest room dimension but not always.
Since you have one located as you noted, I suggest measuring 16 inch and 24 inch in both directions and try drilling there and you should hit them with one of the holes. You can also 'thump' with your knuckle or object to get listen for the difference between the solid sound near a joist and the hollow sound between joists to get an idea of where they are located. And if you get very good, can even locate them fairly well.
Your stud finder might have another setting for thick walls, try using that. Also, the stud finder works only if you start between joists. It senses depth of objects and only reports thicker ones. So if you start on a joist, it will not tell you anything. Also, because of how it works, a stud finder does not give good results for walls or ceilings that have very rough surfaces or surfaces that change in thickness.
Note that you need to attach to the middle of the joist so simply hitting a joist with your drilled hole is not really enough. Once you locate a joist you can drill on either side until you find one of the edges, mark it, and then the other edge is usually 1-1/2 inch away (it can be more for manufactured joists), mark it, then attach to the middle of the two lines which is, of course, the middle of the joist.