Dayton stainless sump pump

I just installed a new sump pump with tether. I adjusted the tether and the pump kicks on when tether raises.
The pump runs but does not pump water. I took it outside and hooked up a temporary 8 foot line to it and it shoots well above the 8 feet. I reinstalled it and it still won't pump the water, although it runs.
If I unplug it, and plug it back in, it will pump. I drilled a vent hole below the check valve , so i don't have air lock. The drain lines are all clean and unplugged. The pump will run continuously after the tether raises, but will not pump a drop of water. After I unplug it and plug it back in, it cycles and shuts of as the tether lowers. The check valve is working properly, and installed with arrow up. No impeller clogs . I tested outside with 8 foot pipe and check valve installed and it pumps like crazy.

Why would unplugging it and plugging it back in make it pump?

What can possibly be going wrong?
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