Even though you think you are waiting long enough and being stingy with the water, you are still making some basic watering mistakes.
When you water, water with gusto, trying as hard as you can to make your soil absorb as much water as you can. If you go slowly, the soil can act like a sponge and soak up many times its weight in water. Water with this in mind, until any more water causes run off. You can NOT overwater by watering too much at any one time... overwatering is instead watering too often.
To determine when to water next time, forget the knuckle method, it only tells you what is happening at the top. You need to wait until ALL the water is gone from that container... all the way to the bottom.
The best way to do this is by using the lift method. Take a container just like what you are using, and fill it with dry soil just like what you have used for your plant. Lift it up and note that it is as light as a feather. This is what your container should feel like when you water the next time... not just lighter than it was 2 days ago, but light as a feather. Your soil should be so dry that you are amazed that the plant is still happy. You were almost getting there the way that you were watering, but you need to go further to get all the leaves to be able to lift up. Your soil will be sahara desert dry.... mummy's breath dry... and still your plant will not be wilting. Get her there... every time. Your roots will explode with activity, trying to find all the water in your container and you will see the difference up top... guaranteed.