The Ferengie and Breen (it was the Breen that Betazoids couldn't read?) would always be able to sneak up on Deanna because they simply do not register on her senses.
Deanna did sense the Ferengi in the first episode they appeared. Obviously the writers changed their minds and made them completely unable to be telepathically read by Betazoids later in the series.
Since there was no canon explanation to help understand how she could sense their emotions and presence in one episode perhaps it's down to her being half Betazoid. A lot of people assume that her abilities will be halved as she is 50/50. But maybe its not about being half of what she would be if full betazoid. Maybe the hybrid brain acts a little differently to a full betazoids.