Very true, and we saw the same thing with Soldado. To his credit, he'd come out and lead the line well at first, but when the inevitable horror miss would arrive (as it usually did), he'd retreat into his shell and focus on the non-scoring elements of his role for the rest of the game. He did them quite well - closed down, distributed the ball, offered himself as an option for players on the wing/in midfield, and so on...but his destiny was only ever going to change if he kept himself in the box and focused on scoring instead of being a good team player.
However...I think that sort of instinctive goal-chasing is hard to keep doing when you're struggling to put them away. Bobby had an interview with Sid Lowe of the Guardian a while back (I think it was in April last year, when Villarreal played Liverpool) where he talked about how his time at Spurs essentially changed him as a player, and made him far more focused on supporting other forwards than on scoring himself. He talked about it with a tinge of regret (I felt) - said that he arrived at Spurs as a player that thought about nothing but getting on the end of crosses and passes, but left as a far less focused striker. To his credit, he made the most of it at Villarreal last season - he did very well as the support striker for Bakambu, who led the line. But you felt reading that interview that he regretted that it had to happen that way.
That's what a run without goals did to a striker with an enormous pedigree at the top level with Valencia - one who led the team he played with at Valencia, and one who arrived at Spurs at the age of 27 (iirc). It's really harsh on Janssen to expect him to remain single-minded in the face of a goalscoring drought given that he's arrived without much of a pedigree to his name, seems like an amiable and professional lad but one without the leadership qualities that Bobby showed at Valencia, and is still only 22 - he'll want to prove to his teammates and fans that he's worth relying on, even without scoring, and
so he tends to retreat into the non-scoring aspects of his game. At least, that's the way I see it.