15 psi is plenty at a normal idle, but, most Mopars that I have seen have more than that which I think is a result of the oil pump design. Assuming a good pump and normal clearances on the bearings, I would expect at least 25.
Other than the weight of the oil, there is no way to increase the pressure once the engine is assembled...not the idle pressure. The bypass spring controls the high pressure side, not the low pressure side-but, if the bypass is not closed all the way under the spring pressure, then the pump would bypass at idle and the pressure will be low.
I would expect a hv pump to have a lot of pressure, however. Pressure is basically the resistance to flow so that one would expect it to be higher when using a hv pump at low rpm than with a standard pump. Normally, a healthy engine will jump the oil pressure immediately when the engine is revved whereas one will problems will usually lag when blipping the throttle-referring to watching the needle on the gauge
Low oil pressure can result from excessive clearances in the engine. A missing galley plug, too light an oil weight, leak in the pick up tube that allows air to be sucked, a restricted pick up screen whether by debris or too close to the pan, excessive clearances in the pump, etc. Of course, not all oil gauges are accurate as well...
