I thought I'd seen everything... Man you seriously need some help here.
First take the commercials off of the videos that you're asking for help on. I'm gonna click skip on them because you are wasting my time when I'm already helping you.
2nd, everything in the box has been put in backwards except for the riffles... The screen at the head needs to have the open end pointing out so that it washes the fine material through the holes and they have a place to escape and go down the box.
3rd, your expanded metal under the riffles is upside down, those riffles need to point down stream to make an eddie (pun intended
4th) Then Mr. Munster, you obviously have a plugged intake on your pump, it maybe be plugged with anything from leaves on the outside, to the smallest of roots now wrapped around the impeller because you had no screen covering over it.
5th, keep your main hopper flatter to give the material the time it needs to wash before it enters the box.
You will have better luck when the pump starts running the way it did when you bought it but you will probably have to take it apart to clean all of the garbage out of it. 1200gph would be fine for low impact riffles but with these you will need probably 1" to 1½" per foot of drop when it is put together right. Let me know when you have the commercials off of your videos that you need help with and I will watch the rest then.