Twice? What kind of first class spy agency would miss a target, let alone 2 (Skripal and Navalny)?
Murdering someone is not a hard feat especially when you are a government agency with all the resources necessary to do so.
In the case of Navalny, it was even easier as they were "playing" at home.
What's the point of re-using a super duper chemical nerve agent that already demonstrated that it was not a sure deal?
Now it's possible the Russians lost all the KGB experience (remember the Bulgarian umbrella?) and are subpar spies. Why do we need to be afraid of them then?
I believe that when a spy story ends up in the press, it's because we are the targets for propaganda. Else it stays in the shadows.
My theory (which is just that, a theory) is that the US is trying hard to block the Russian gas pipeline to Europe, and this kind of stories allow them to impose sanctions.
That's actually not surprising. Chemical weapons turn out to have lousy effectiveness in terms of killing people. In 1995, a terrorist group released sarin into 5 subway cars during Tokyo's rush hour and managed to kill a whopping 12 people in the process. A single grenade would likely have killed more people. Admittedly, sarin isn't the most lethal chemical agent, but it is one that was actually used in chemical weapons programs, and the stronger agents implicated are only like 10× more lethal. The notion that "military-grade chemical agents" are "supposed to kill full cities" is propaganda about the effectiveness of chemical weapons; it's not a realistic assessment of their actual capabilities.
At the same time, there's also a world of difference between a targeted killing of a single individual versus a generalized killing of a large mass of people. The mechanisms you need to use to achieve this are different: in one case, you want to deliver a more spiked concentration of chemical to one individual, preferably in a manner that allows it to take effect when those who delivered it are nowhere near. But killing an entire city requires you to have extremely fast diffusion with extremely high initial concentrations, so that you can kill people before they start to run away (or don protective gear). That someone screws up the dosing when trying to use it as a targeted weapon is easily plausible.