WE have frost forming on the underside of the roof decking and on the nails there seems to have adequate insulation R-30 proper vents and ridge vent
Ridge vents are the best venting for a roof because they give continuous venting across the roof. Other vents, such as mushroom vents, can leave hot air trapped at the peak of the ridge. When working properly, airflow is drawn in at the soffits, up through the attic and exiting at the ridge vents. This keeps your roof cool (preserving the life of your shingles), and keeps your house cooler in the summer, because your attic isn't heating your house from the top.
Given that you have ridge vents, you should check that you have continuous venting in your soffits and that they are not blocked. If this is good, you should check that your attic insulation is not blocking the airflow from the soffits (as noted by Johnnyz).
The other thing to consider, if the ridge vents were retrofitted, you may have gable end vents that should be blocked up because they interfere with the airflow from the soffit->ridge.