Plus one. Get a whip line between your unloader and the rest of your setup. Btw, what’s that ball valve for, anyway?
Also, you’ve got your quick connects “backwards”. Male end should lead, and female receives at the next connection. It wouldn’t be what’s causing this issue, but it’s bugging me nonetheless.
If you decide to switch everything around, just make sure to do it with the connections on the injector, as well. (You have checked which way the arrow points, right?)
What is your draw rate like with the gun off? If you’re getting a poor draw rate, you might have some obstruction in the hose, like a crushed/kinked spot from being run over. Then the gun adds just enough back pressure to stop it drawing entirely.
I’m also a fan of trying a fixed injector. If you can locate a GP hi-draw, they’re a good, cheap option.