I don't know many teachers from Tibetan schools that's why I ask this question.
Is it common, or normal, that a highly educated lama teaches to his Western community some sadhanas, gives the necessary empowerments, but otherwise gives no lectures on general Mahayana nor on Tantra? What could be a reason for this?
Since I haven't been to many sanghas besides my own, I can't comment much from personal experience; however, I have heard many people on Buddhist forums say that when their lamas teach about general Mahayana topics that very few people show up. Perhaps, some lamas got tired of giving teachings which no one would listen to.
This doesn't apply to my lama, however, because he talked for over a year about Shantideva and he's always talking about the Six Paramitas and the Eight Worldly Dharmas. This year, he's talking a lot about death and rebirth.
Also, sometimes visiting lamas give general dharma teachings as well as empowerments. The visiting lama who I received a Vajrasattva empowerment from also gave a series of teachings about the Heart Sutra.
Although in cases where a lama is teaching people who are already Buddhist scholars, he or she might only teach advanced meditation practices because he or she assumes that the students already know the basics.
Tilopa was the epitome of a silent teacher, since he remained silent for a whole year a time and then taught Naropa advanced practices after Naropa supplicated him and passed a difficult trial.
Another possibility for not teaching general Dharma might be that the lama feels that the students might not be receptive to it yet, and might feel that it's simply better for students to develop and open their minds through meditation before they try to confront difficult concepts like emptiness.
Some students may approach Buddhism in the same way as they approach yoga: as merely a nice way to develop their minds and bodies a bit, and Buddhist teachers have to think of what the best way to benefits these types of students is.