Fertilization of rhododendrons should be done with fertilizer formulated for acid-loving plants. Measuring the soil pH with a soil test or kit is important because rhododendrons are acid loving plants, and do best in soil with a pH of 5.5-6.0. If the pH is higher than this you need to add sulfur, either in the form of elemental sulfur or a sulfur compound. Aluminum sulfate is not recommended as the aluminum can build up in the soil. The availability of other nutrients can be dependent on pH. Right after blooming is a good time to fertilize as the plants put up new growth. This article has good information on growing rhododendrons,  Azalea and Rhododendron Care and Culture. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/fs12.pdf

The next step is to look at your plants. Are they growing well and blooming. Do they show signs of deficiencies, for example light green leaves may indicate a lack of nitrogen, yellow leaves with green veins can indicate a lack of iron, though this is often because the pH of the soil is not acidic enough. Provide a balanced fertilizer, either organic (slow-release) or chemical. Do not fertilize after mid summer because it will stimulate soft green growth subject to winter damage.