To piggy back., Expansion tank has a tire shrader valve., make sure its only air in their and has not filled with water. If it has water, needs replacing., if its air only., drain the expansion tank from the water side. Fill the Air section to match the boilers pressure under normal operation, then reinstall/fill with water.
Whats the psi set point of the relief valve?
Lastly, Make sure your gauges are accurate and not faulty reading lower than normal. Is their a Pressure reducing valve attached for the makeup water autofeed?