They aren't errors. They are warnings. I suspect you have compiler warnings turned down or off.
Go to the "Preferences..." page and set "Compiler warnings:" to "All". When you compile again you will get helpful warnings that point out where you are doing something that doesn't look quite right to the compiler. Warnings point to code the is not a syntax error (so it WILL compile), but it is fairly likely that it is not doing what you think it is doing.