I don't think that it's a pump issue. The pump is not self priming. It requires a flooded suction. I suspect that the way it's plumbed allows air to build up in the pump.
If the booster pump suction Ts into the underground return line, there might be insufficient pressure to move water through the pump.
The T might even create a venturi effect, which can lower the pressure even more.
You need to put the booster pump intake on the return line in a way that water continuously flows through the booster even when the booster is off.
If the booster pump suction Ts into the underground return line, there might be insufficient pressure to move water through the pump.
The T might even create a venturi effect, which can lower the pressure even more.
You need to put the booster pump intake on the return line in a way that water continuously flows through the booster even when the booster is off.