During a pump startup, the pump appears to take several seconds after flow the operating rule pattern before it finally starts deliver flow.
This is likely related to the pump being unable to overcome the downstream head already present in the system until the relative speed has increased to a certain point. You can also see a similar effect in the pump startup sample model that comes with HAMMER.
In cases where there is a downstream tank or reservoir, the downstream boundary may be providing head to the system. The operating rule for the pump may have the pump reaching full speed after a number of seconds. Between the start of the pump and when the pump reaches full speed, the relative speed factor will be increasing, but will be relatively small to start.
The following suggestion can help illustrate this. Compute the initial conditions for the model with the pump's relative speed factor set to a small value, such as 0.1. When I do this, there will likely be a message that says the pump cannot deliver head or flow. That is basically what is happening in the model. Just after the pump starts up, the relative speed factor is still ramping up and the resulting head and flow is not enough to overcome the downstream head. Once the relative speed fact gets high enough, it can and the flow starts through the pump.
Modeling a Pump Start-Up Transient Event in HAMMER