According to Heidegger philosophy has never really asked what we mean by "Being"
Well, you raise a whole series of fascinating issues in your question. I'll just focus on the claim Heidegger makes, and not direct myself to either Fichte or Kant.
What does Heidegger mean in claiming that the question of the meaning of Being has rarely if every been asked? I wouldn't say that he means that the question has been 'supressed' -- in the way free speech is supressed in a totalitarian regime. Rather, he means that the question has always been raised only with respect to some limited frame of reference, where that frame is determined by other philosophical commitments. A theological frame of reference understands Being only as either creator or created; the frame of reference of mathematics yields a conception of Being as substance; a technological frame of reference understands Being (including the human) only as the availability or otherwise of resources; and so forth.
The other point worth making is that although the above discussion makes Heidegger sound as though he is profoundly dismissive of previous philosophers. Nothing could be further from the truth. The vast majority of Heidegger's work comprises very careful and scholarly (though often enough controversial) workings through of texts from the history of philosophy, from the pre-Socratics through to the relatively recent. In each case, the name of the game is to find those moments when the question of Being itself is indeed asked, but then gets immediately covered-over or interpreted away according to the demands of the frame of reference (as we termed it above). This is what Heidegger, in an unfortunate choice of words, calls the 'destruction' of the history of ontology. This procedure is a kind of reading philosophy against the grain, so as to bring into relief questions that were nearly asked, and types of thinking that were not, but could be, used to raise the question in the present.