The easiest way to describe it is to gently take one apart and you will see how it all works. If you have the ones I think you have, with the toggle on the top open (vertical) a sprung loaded pin will be retracted from a hole above the little sight glass, allowing the oil to drip. If this isn't happening you undo the locking knurled wheel under it and turn the cap to gently move the toggle upwards until the hole is open. If you do it the right amount it will shut when the toggle is flicked over. I was once told by an older machinist that the "right amount" of flow is when you are just about getting bored and wondering if it will ever drip… and then it does. obviously that is very subjective!

It could be that the lubricator is blocked, or the mesh at the bottom is gunged up. Again, you will only know if you carefully take it apart, laying it all out in order. It will soon make sense. Just be careful not to damage the glass.