It's hard to tell much from the picture, but the plant itself is obviously alive, so there's plenty of hope. Don't do anything to the soil unless a test shows that it is not acidic enough. A too high pH would not in itself prevent blooming.

The most important question is does the plant set flower buds in the late summer/fall which then fail to open in the spring? If that is the case, then winter kill of the buds in your very cold climate is the most likely explanation. In this case, anything you can do to protect the buds from drying winter wind and sun can help. Unfortunately there is nothing to be done about temperatures below the point that buds can survive.

If no buds are set: The most usual causes are: a rhododendron that has not yet matured enough to flower. Too little sun in the summer. Excess nitogen which promotes vegetative growth at the expense of buds. Drought in late summer/fall.

Zones 4a and 4b are at the limits for growing broad leaved rhododendrons. If this is one of the "iron clad" varieties, I'd guess that it is just not yet mature enough to bloom.

Thank you for responding with your advice. I think you're right about the winter killing the buds, as I do see buds on it every year and really thought they'd open. I will try protecting it this winter as I haven't in the past, and see if that makes a difference next year.