What makes ears feel full?

Full feeling ears. Nasal congestion for a cold or nasal allergy symptoms, fluid in the ears, airplane descent or descent from high altitude, are some of the things that can make the ears full. There is physics involved, but simply put, ear feel full or plugged when the eustachian tube that is responsible for equalizing pressure between the middle ear space and the "outside" fails to do so for some reason or other.
Possibly the jaw. A lot of times when patients talk about pressure in the ears it has nothing to do with the ears at all. Rather sometimes "your pressure" can be due to muscle spasm around the jaw joints. The jaw joints happen to be right around the ears and when the muscles that move the jaw are in spasm the sensation of pressure can result. This is common in people who clench and grind their teeth or chew gum.
Pressure. Partial blockage of the eustacian tube which runs from the side of the back of the throat into the middle ear. When this happens it creates a sensation of fullness. You can try relieving this by thrusting your lower jay forward and your head back, or by holding your nostrils and blowing against a closed mouth (but be very careful with this), or use a nasal spray or decongestant or a sinus rinse.