from what i understood he wasn't able to read his mind fully. he tried, but with the guy being half romulan he had certain mental abilities too that he was able to hide his specific thoughts? h
Romulans don't have the telepathic abilities that Vulcans do. They can't mind meld, for example. (And Tarses was only 1/4 Romulan, in any case.)
Then perhaps that 1/4 Romulan brain structure and 3/4 human (maybe, because we don't know that the rest of his heritage is pure human) prevented a telepathic probe. Nothing active on Tarses' part, just a natural thing.
Also, was it ever said that the Admiral's assistant was a full Betazoid and not 3/4, or 1/2, 1/4 or any other fraction?