Is it true that when a waiter recommends a dish it's because that dish isn't selling well?

I don't think that is the case. He should recommend something that you would like so that you will give them a big tip at the end.

Not always. Sometimes they recommend their favorite dish. If a dish isn't selling well and you order it and don't like it, he's not going to get a good tip.

I am sure it is sometimes true.
But I worked my way through university (including full-time during summers) by waitressing. And I waitressed in a variety of restaurants, most of them high-end establishments.
I was never instructed to push a particular dish.
When someone asked me for a recommendation, I suggested whatever people seemed to order the most (and, naturally, didn't complain about).

I think that what is most significant about my experiences is that I was never ASKED to push a dish. and I worked in 8 different restaurants over those years (5 of which were higher-end restaurants).

no not at all. sometimes that happens to be the waiters favorite food. when there is a special on a certain dish, THAT means they are trying to sell more of a particular dish to get rid of it.
Of course this all depends on the restaurant. Sometimes the special is designed to get people in the door. If hubby loves the special and brings the wife too, the restaurant sells the special plus what ever the wife eats. AND maybe dessert too.

Usually it's because they have too much of it or it's the "special" of the day.