If there has been settlement, which will draw the liner downward, it will pull on the liner where it attaches to the bulkhead fittings. If it is in the pond and not the waterfall, I would suspect the bulkheads. Bottom drains do not have forces pulling on them that will pull the liner loose, like the bulkhead fittings. I had a situation and was able to find the problem putting my finger below the bulkhead and see that the liner was not even in the bulkhead.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard