So these are just guesses, but some aspects of your design may be causing issues. First, as you mention, these typically extend directly downward from the bottom, making the valve the absolute lowest point in the system, so the media falls straight down into it from the bottom of the hopper. In your implementation, there is an angle that would introduce friction as the media would have to slide diagonally at a relatively shallow angle along the tube to get to the lowest point.
Then, you're introducing a right angle, which is another obstacle the media has to navigate before it reaches the point where it gets "picked up".
Finally (and I'm guessing this is the biggest issue) the media has to go up hill for several inches before it reaches the point where it enters the hose, and that "up hill" portion is at a larger diamer than the hose, which means a pressure drop is created. Because of the up hill portion, you are effectively adding an extra section of pickip hose that is at a larger diameter than your final hose, then introducing a restriction at the point where it is necked down to the final diameter. With the typical design of these, gravity would be assisting the media falling all the way down to the point where the hose starts. Also the typical design uses a reducing coupling to create a smoothe transition between the pipe diameter and the hose diameter. You're using a bushing which creates an abrupt ans interrupted transition which I'm sure is hurting flow.
Generally, you have several more fittings in your design than typical, each of which creates a barrier to smoothe flow, you simplely have more length of pipe in your valve, and you're not fully leveraging gravity.
Here are a couple pics of one I made which I consider pretty typical.
If I were to try to modify your design, I would replace the initial leg you have coming down from the flange with a tee (close ******, tee, close ******) install your valve on that tee, then install your bushing (or, ideally a reducing coupling) on the end of that and screw your barb fitting into that, then clamp your hose on so everything is still flowing downhill when it reaches your hose.
Edit: I realized I missed another critical point. Because the lowest point in your setup is the elbow, and because the valve opening is above that, when you open the valve, air is coming in through the valve and then going straight out the tube without taking any media with it. Typically, the spot where air comes in the valve is full of media, so the media gets swept up with the rushing air into the tube. Again the uphill leg is what's killing you.
