Here are some examples of feedback your viewers may think, but are too polite to say…
“Your house smells” … of dogs /cooking / smoke…  
“Your house is dirty.”
“Your clutter tells us you don’t fit into your home so we probably won’t fit either.”
“Your rooms are bland / boring.” (Or, “Your room colours are awful.”)
“Your garden is a mess, it must be really hard to look after.”

None of these things would take long to cure if they bought the house, but the chances are they won’t buy it anyway.  So, clean up, tidy up, and give your home more of a showhome feel.