Why did the Neutrality Acts prove ineffective in the conditions of the 1930s?
I am not at all sure that it is correct to say that the Neutrality Acts did prove ineffective. At the end of the 1930s, the Neutrality Acts were all still in effect (though FDR had said he regretted signing them). The last Neutrality Act was passed in 1939, so Congress clearly thought they were still useful. Huge majorities of Americans continued to believe in the idea of neutrality. In addition, the Neutrality Acts arguably helped keep the US out of war during the whole decade of the 1930s. Looked at in this way, the Acts were effective. They fulfilled their purpose, which was to keep the US out of wars.
However, you could argue that they were ineffective because they ended up working to help the "bad" side in some wars of the '30s. For example, when Italy invaded Ethiopia, refusing to sell arms to either side was the same as helping Italy. This was because the Italians didn't need arms whereas the Ethiopians did.
You could also argue that they were ineffective because some of the conflicts were not officially wars. The most important case like this was the Japanese invasion of China. Neither side called it a war, so the Neutrality Acts did not apply.
However, I would argue that neither of these really does show that the Neutrality Acts were ineffective. I would argue that they fuliflled their purpose and kept the US out of war throughout the '30s.