You can check soil pH with bromocresol green indicator. This is an inexpensive way to do your own soil pH test. Take a one tablespoon soil sample, and mix with enough distilled water to make a slurry. Run the slurry through a paper filter, such as a small coffee filter, and collect one teaspoon of filtrate. Add just enough indicator solution to see some color. Yellow means pH is on the low side, at 3.8, and blue means pH is on the high side, 5.4. If the solution turns green, pH is 4.3, 4.4, 4.5, 4.6, or 4.7, and this is ideal for growing rhodendrons. Soil pH can be lowered with agricultural sulfur, spread around the shrub, and covered with mulch.

If it is a pH problem (lack of acidity) the leaves will be yellow with green veins. This is a very reliable way to detect chlorosis.

Light is the most important component for good bud set. General vigor is important, and using some HollyTone now my help for next year. Never use more than half the amount recommended on the package. Always use a good mulch layer.

My soil is very low in phosphorus, so I need to add super phosphate. It helps with bud set. Make sure the buds aren't dying during the winter, or being eaten by squirrels.

Here is a link that might be useful: How to grow rhododendrons and azaleas