If the indicator light is on, but there are no bubbles, there is something in between the two “screens” of the oxygenator and this is preventing the reaction that makes the bubbles take place.
Typically, hardening water or something like calcium/lime build up is occurring in between those screens, so you will have to clean them out. If found early on, you can get these minerals off by using a soft toothbrush, baking soda, and vinegar mix. In some cases, you may have to use a mineral deposit remover like what you would use in your bathroom or shower to remove mineral deposit.
Always be sure to thoroughly clean out your livewell after using chemicals for cleaning and before putting the boat back in the water. Once you have cleaned the screens, fill your livewell so the oxygenator is submerged and test its performance.