when you build a waterfall,you should take liner and put it under the entire waterfall and cup the edges so no water can leak out,do the same front to back.

I would feel around the edges of the falls to see if any of the dirt feels damp (or damper than it should be.

If you can move your hose, I'd take it out of the waterfall and have it run directly into the pond. If no leaking occurs, you rule out the hose.

Then I'd move the hose to different sections of the falls (lowest tier, then further back, ect ect). This could help you isolate where the water loss is occuring (or most of it is occuring).

My guess is you're losing it through the rocks and mortar, if your falls doesn't have any sort of liner. That's bound to have pathways for water to escape.

Before you do any work on a leak, did you know that when the falls is off all the water runs down to its lowest point and out the overflow. It is normal to have a sudden drop in water when you start the falls. This is one reason most people run them 24/7. Start the falls and add water till full. Then see if the pond holds its level
If you do loose water go back to the first two posts.

Thanks for your advice, We have a lot things to check on which I wasn't aware of before.

Regina