Thirty years of experience has taught us a pretty fool-proof way to make sure our filtration can do its job effectively: pull water from one place, and push from everywhere else.
That philosophy is why we don't recommend under-gravel drains. When you have a force pulling water from under your pond, that force is fighting against your skimmer and filter trying to do their work on the surface. This method is also why we recommend, if you have a particularly large pond in need of more than one skimmer, placing all your skimmers next to each other on one side of the pond. Putting them at opposite ends can create a kind of Bermuda Triangle in the middle of your water feature where the water is prone to stagnate.
But what about areas of the pond where the skimmer can't reach? That's where the pushing part comes in. Jets do a great job of forcing water in the right direction.
So remember, the bottom line when it comes to filter placement is this: pull from one spot, and push from as many other spots as possible.