Your Hoya likes to dry almost all the way down between waterings. Always check the soil before watering. Water thoroughly and evenly around the pot until you see water flow out of the drainage hole. Be sure to discard any excess water after a few minutes.
If the leaves dropping are dry and crispy, then your Hoya’s soil most likely got too dry. If the leaves are mushy or yellow, then it was overwatered. If you notice either of these symptoms, check the soil moisture to see which issue it might be.
Your Hoya prefers bright indirect light. While Hoyas can adapt to low light areas, their growth may slow. If placed in too low of light, yellow leaves may develop. Bright indirect light can be found in places close to an east-facing window or a few feet back from an unobstructed southern or western window.
Hoyas usually prefer warm humid environments. If it gets too cold or your Hoya is placed in a drafty area, it may respond by dropping its leaves. Make sure your plant is not in a drafty area or in the direct path of any open vents.
Low humidity and dry soil cause leaves to droop and brown on their edges, later followed by entire yellowing, browning, and shedding of leaves. If this happens, your Hoya will appreciate a boost in humidity from regular misting, a humidifier, or pebble tray.