I don't think the system pressure is an issue at all...effectively from one side of the pump to the other, it starts out as zero. If the tankless has a flow restrictor, the pump will have more trouble moving the water. Depending on how the pipes are layed out, it could be recirculating through another loop. Some valves can fail and have an internal cross-over. If the water was moving through that valve rather than back to the tankless unit, it would never see the flow.