Do check valves require a minimum PSI to even permit a forward-flow (not a back-flow)

I have a very low flow, low PSI application, 1 - 1.5 PSI, and I am seeking an appropriate check valve. I recently bought a check valve, and after blowing in to it, I am finding that it does NOT permit air to flow in either direction. Do I have a defective valve, or is the minimum required pressure to flow through that valve not present? If check valves do require a minimal pressure to flow, does this mean that my system will automatically experience a loss in pressure after passing the check valve?

The flow must overcome the weight of the check valve disc, which is minimal, however you are not going to raise the disc by blowing into it.
You cannot generate enough force with the force of the air generated from your lungs. The air does not have sufficient density.
When the check valve is submerged in water it only takes a very minimal flow to raise the check valve from it's seat.

err no more than 0.5 psi ABOVE the 1-2 PSI (so 2.5 PSI) which tries to push its way back every few minutes for 20 seconds or so..

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It depends on the check valve design. Some have a spring to help them close and other rely on gravity to do it.

Also the size affects the cross sectional area of the disc or ball and that sets the force that any pressure differential can build.

You may need a diaphragm type check or one that uses a short piece of soft tubing that collapses when the flow tries to reverse.

Yes indeed they do require a base forward bias pressure differential in order to operate. There needs to be some pressure to push the barrier against the spring, and open up a passage for flow.