Low Pressure Sanitary Sewer - Negative Pressures

I'm modeling a low pressure sanitary sewer. The goal is to see how the system reacts as homes/grinder pumps come online. The issue I'm having is with only a single grinder pump operating to start in a steady state analysis, the system HGL goes negative part way through the system. See engineering profile attached. When I model the system with 3 grinder pumps online at the same time, there is enough flow so that the HGL is positive. It's as if the the single pump is trying to fill the system and cannot. Any insight into why SewerCAD reacts this way would be helpful. I have also attached the model. Currently the additional pumps controls are set so the on/off HGL settings are the same so that they don't come online.

Profile Report_Engineering Profile - Profile - 1 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdfLow Pressure Model.zip