That means your marketing is working but people aren't seeing the value. It could be your marketing is creating false expectations. If a property boasts a view, but you can only see anything interesting from the master bedroom closet, it won't live up to expectations (I've seen this example).
If your pictures make the rooms look bigger than they are or the home looks different for showings than it does in the pictures.
Apart from that, price is the next most logical reason. It could be both. If you've had 21 showings or been on the market 21 days without an offer, something needs to change.