What might cause a low End-Sytolic Left Ventricular Pressure Working Heart Experiments?

If low End Systolic Left Ventricular Pressure (LVP) is being measured during a Working Heart Experiment, the first thing that should be investigated is the preload and afterload pressures that are being applied to the heart.
Preload is set by the height of the atrial bubble trap, and should be approximately 5mmHg in most animal models.
Afterload is generally set by the height of the compliance loop, shown in the image below. Regarding Afterload, 50-80mmHg is normal, depending again on animal model.
Both of these should be checked against previous literature to find the proper settings for your animal model.
Another troubleshooting step would be to ensure the proper use of the Compliance Loop. The compliance loop can greatly affect the developed pressure of the ventricle at the end of Systole. The bubble trap portion of the compliance loop should have a 2mm bubble very close to the outlet of the aorta to mimic the compliance of the arterial vasculature. If the bubble that the heart is beating against is too small, this may model a very stiff vasculature, and cause an increase in resistance, and an increase in End Systolic LV Pressure. A Large bubble would model a very compliant vasculature, and may result in low End Systolic LVP.
One final thing to check in this experiment to remedy low End Systolic LVP would be to ensure proper atrial filling. In some circumstances, high resistance in the filling line may cause enough restriction as to limit the Atrial Filling Volume. If this is the case, low End Systolic LVP will be the result.
If your preload (atrial bubble trap height) is set properly, a test can be performed to ensure proper atrial filling. First, without a heart attached, measure the amount of fluid draining from the atrial cannula while the fluid is free to empty from the bubble trap. This should be well in excess (2x recommended) of the cardiac output expected.