Errors beget improvements
Morag, this is a collaborative site. It’s not perfect. But it often produces excellent results, and the errors are part of how it works. When you see a flaw in another user’s answer, that might give you an idea for a better answer. So write a new answer and post it! If the best answer posted so far is technically correct but poorly written, or written at a level beyond that of the questioner, post a better-written answer! When you see flaws in a question, you can edit the question. In comments, you can point out flaws in other people’s answers and suggest improvements. Your answer might trigger someone else to think of an even better answer, and on the cycle goes.
Those are constructive responses to errors. It’s often much easier to get an idea for improvement from seeing someone else’s error than starting from scratch, and this cycle of improvement often leads to some extraordinarily well-informed and helpful writing for people learning English as a foreign language (and indeed all throughout stackexchange.com). The cycle of errors begetting improvements is also how Wikipedia works.
Living with imperfection
The process is not perfect. Sometimes, a questioner accepts a very wrong answer. Sometimes a questioner accepts a merely okay answer when someone else had written a truly excellent answer. Sometimes a good answer gets downvoted. Imperfection is the price we pay for amateur, volunteer participation and the cycle of improvement.
When a good answer doesn’t receive the public recognition it’s due, or gets downvoted or criticized, it can be a little infuriating, especially if you’re the one who wrote it. When that happens, the best thing to do is remember that the system is imperfect and move on.
It doesn’t help to insist that you’re right or post fulminations about the motives or level of knowledge of the other contributors. Sour remarks only distract from the relevant content of your answer, making it less useful for learners who might come across the page later. Posts containing personal disparagement will get deleted, because they degrade the tone of the conversation for everyone.
If you really want to air a grievance, come over to meta.ell.stackexchange.com and point out the problem as specifically and factually as you can, and either ask for advice on how to handle it yourself or, as specifically as you can, ask for someone else to take action. Folks here disagree a lot, but they’re friendly and—of course—they’re very good at responding constructively.
If you can’t tolerate disagreement or imperfection, stackexchange.com might not be for you. If you can, it’s a great place to learn, contribute, and be part of something greater than yourself.
with evidence in a pleasant, polite manner? It looks like the pot calling the kettle black to me.