One way to check for systematic errors is to run experiments of a different design that should give the same answers. Scientists often do different kinds of experiments to cross check their results. Another way to locate errors is to have an independent investigator repeat your experiments. Others should get the same results you did. Your results can be invalid if your variables are not independent of one another, and you have not noticed this.
Variables are independent if they produce their effects separately from each other.
In other words, changing one variable does not affect changes produced by another variable. No matter what happens, you will learn something. Even if your experiments don't answer your questions, they will provide ideas that can be used to design other experiments. Knowing that something didn't work, is actually knowing quite a lot. Unsuccessful experiments are an important step in finding an answer.