What can make an echocardiogram hard to read?

Several causes: COPD (lung disease) is the biggest one. Extreme obesity as well as extremely thin people also are difficult to image.

Many Many Things: I do echos & have worked with many techs; heart echos are rarely easy; very dependant on the machine, person, tech, reader & commonly “read” rapidly, accepting inaccurate measurements & looking only for marked abnormality (the standard medical algorithm), not excellence of heart function. Sound will not penetrate ribs or lungs, images obtained usually off optimal angles; many options for ↓ quality.