It went from almost too easy to incredibly difficult; https://en.wikipedia.org/wiki/Johann_Georg_Elser

After Elser failed bomb plot, the obvious embarrassment on the SS, Gestapo, and SD's behalf meant from then onwards security checks at all of Hitlers events were rigorous. Attempts came from civilians after this point, but always failed or were intercepted. The only blind spot was the dissidents in the army which as we famously know very nearly succeeded; https://en.wikipedia.org/wiki/20_July_plot