Amplify for low voice in podcast

Hello,

I am editing a podcast yet one of the voices has been recorded lower then the other. I used the Blue Yeti but app even the 50/50 function records one side better then the other.

Now I selected the sections of the lower voice to make it louder but everytime I want to amplify to make that particular section louder it mutes the whole line. I included 2 screenshots. Can anybody help me? Thank you.

I bet that if you tried playing that track before amplifying it, it would play as silence.
My bet is that this has nothing to do with the Amplify effect, but is to do with the sound “disappearing” and that applying any effect causes the waveform to become a flat line.
Am I right?