Why I have a problem with All Our Names

I've been posting on the opaque nature of the two main characters, Isaac and Helen, in Dinaw Mengestu's All Our Names, and let me give two examples. First, Isaac: In the Isaac-narrated chapters the young man, then known mostly as Langston but by and large nameless, is drawn to a house where a group of revolutionaries plan a coup to overthrown the government of Uganda - men keep arriving a limousines, armed guard spring up all over, a cache of weapons and ammunition arrives hidden beneath crates of bananas, all very good, but we learn little about the coup of the men involved with it because the narrator himself knows nothing - he's in the dark, and therefore we are too. This could be OK except that there is no significant tension throughout this whole section because we know - as the novel is narrated in two strands separated by maybe about a year's time - that the character, later to be known as Isaac, escapes Uganda unscathed and begins a new life in a U.S. college town, where he becomes a couple with the other narrator, Helen. As to the Helen chapters, she observes Isaac, knows that he has a secret past life - but that doesn't matter much to us as readers because we know about his background already. What maybe could matter to us would be if Isaac (and Helen, too) were in any danger because of his past political associations. There are opportunities for Mengestu to turn up the volume on this plot: Helen, for example, late in the novel meets Isaac's U.S. mentor and confidant, a man named Henry. How much stronger this novel would be if Henry gave Helen a warning - be careful of this guy, or be careful around him - and Helen had to make a difficult decision. But, no, he tells her she's the best thing that's happened to Isaac - and her only worry is that his student visa will soon expire. Mengestu is a very kind writer; he really seems to care for his characters - but as a result he's reluctant to put them into any jeopardy or to let them make bad or dangerous decisions. He does a great job at setting up a plot and characters - the first Isaac chapters in particular were very promising - but he doesn't seem able to sustain the dramatic tensions that we expect, need, in a plot.