People don't really understand what the UN can and can't do.
It's not and never was some kind of independent military force that can make countries behave. If you expect the UN to be that then you're not being realistic. The organization does do some good, but when it comes down to it, it can basically only do something if all the member countries agree. Blaming the UN for what's happening in Palestine is misguided, if anything blame the countries that are in the UN, not the organization itself.