Hidden Wire Detector, Oscilloscope Measurement
You're applying a rising voltage to the base-emitter diode of a bijunction transistor.

What you observe is the expected behaviour of a diode in this situation. Look up "diode I/V diagram":

Current through the diode is exponential to voltage, and at some point, it really conducts very well, so that the base would have to source much current to significantly increase the voltage.

However, where should that current come from? Your antenna isn't a perfect voltage source, so it simply can't increase the current any further. 

The problem is made worse by you attaching an oscilloscope: Now that draws away another part of the already very small current.

So, this is all working as expected! If you need a more sensitive amplifier, you'll have to build a better amplifier than what looks like to be a low-gain silicon general purpose transistor based one without proper biasing.

Another word on your reference: Youtube really has a terrible ratio of bad electronics videos to good ones. Maybe look for another source of information. This isn't a great circuit, and a very similar one was in my first exam about transistors and diodes with the question "Why doesn't this work very well as amplifier for alternating currents". It was an entry-level question.