I think it's been stated somewhere that not all Betazoids have equal telepathic capabilities. Lwaxana was regarded as exceptionally gifted in telepathy, being able to read people's thoughts easily. Keep in mind, I don't quite recall where I heard this... It may have been in a book, which would technically make it non-canon, but it would seem to fit. I think Jenestra would have read Tarses' precise thoughts if he could have, and Lon Suder never displayed any particular affinity for reading minds (I believe he actually states something along the lines of "Most Betazoids can read other people's emotions, but I can hardly even read my own").