I'm not sure, but I think you are using the wrong mode on the prospecting pick. The prospecting pick has two modes, which you can switch with [F]. The first one shows you if there are any ores in a 6x6 area (you can change the size somewhere, I think). This is great, if you know where a deep ore vein is, but not exactly helpful, when finding a vein in the first place. What you need is the second mode. After you take 3 rock samples (break three blocks with the prospecting pick, each 4 blocks apart), it will show you how many ores there are in your area (also reaches to the world bottom). Like this you can pinpoint larger ore veins. Then, as you're digging down, use the first mode.
I recommend watching a guide on YouTube, like this one I found -> https://www.youtube.com/watch?v=-CBlbhvq43I
