marmel is right.
Your body tries to keep things in balance, constantly.
Let's say your natural test level is around 400ng or something. You take a booster, and let's say it works and raises it by 40ng. Your body now has 440ng, however, since the natural range was around 400ng, the extra test triggers a negative feedback effect and scales back endogenous test production to try to normalize your test levels back to 400ng. There is a net change of almost nothing or nothing.
The only way to overcome this is as marmel said, you need supra physiological levels, or in normal terms, way above what your body can produce. However this triggers the negative feedback effect so much that your endogenous production is shut down, and you need to restart it (hence the need for PCT). The only things that will really shut you down are steroids or pro hormones. The effect of test boosters is too small to make any meaningful change.
If it worked it would be illegal. Only some pro hormones are legal, but the components needed for PCT are illegal anyways. If a test booster proved to be working, it would be banned once the government found out it was working.