What Causes Fluid Behind The Eardrum Affecting The Quality Of Hearing?

My mother has had problems with her ears ever since having the flu she took atb s 2 different times and tried flonase decongestions she had a scan and dr says she has fluid behind eardrum he wants another scan to study it Moe before sending her to Ent this is also affecting her hearing what. Can be done for this and why another scan