As someone mentioned above you have to be 1 level higher every time to be able to see the node and mine from it.

To clarify:

When you reach level 5 for example you will have to keep mining the level 5 node until you are level 6. Once you are 6, you can head to the level 10 nodes and xp on those till you reach 11.
At 11 you can you head to level 15 nodes until you ding level 16 etc

Also be sure to check different areas as some areas have higher and lower level ores on the nodes.

So if you are level 17 for example you will be mining on level 20 nodes. Of course some items/ores will be level 20 in your list and as you will see there will be various different levels to choose from in a node. In this example you're level 17 so i would mine either the items in that node that are lvl16 or level 18 so as not to miss too much.
Of course your gear does help a lot and depending on that and the % of success is what you normally pick to mine.

Different people do it in a different way, this is just what i did myself and just an example.

Hope that helps.