Answer: Unfortunately, based on your description I am not certain what is happening to your rhododendron. It is not unusual for a newly planted rhododendron to skip blooming for a year or two while it becomes rooted and established in the new location. The fact it is growing is a good sign.
Meanwhile, it is possible that the soil is not acidic enough, thus causing the yellowing. To check that you would need to run some basic soil tests. It is possible that it is being overwatered, which would cause yellowing at the base of the plant first. The soil should be slightly damp like a wrung out sponge, not sopping wet or saturated. To know if you need to water, pull aside the mulch and check down into the soil with your finger. If it is still moist, do not water yet. It is possible it is reacting to a "hot" mulch such as horse manure with too much nitrogen or wood chips that were too fresh or to being overfertilized. Or, it is possible there is a disease problem.
I would suggest you consult with your local county extension to obtain a more specific diagnosis of what is happening and based on knowing that, determine how to proceed. They can also help with soil tests if they suspect that is the culprit.
Q&A Library Searching Tips