Is the entire run of the hose to the pump intake and the entire run of the hose from the pump discharge within the pond basin?
If not, then you probably have a hose working loose at one of the connections, or a cracked hose in need of replacement.
If your entire water circulation system is within your pond liner, then you probably have a problem in your "upper basin" above the waterfall. Look for a place where the liner edge has slipped down to the normal water level with the pump running, or for a tear or crack in the liner right near that level.
Good luck finding your leak!
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