Without watching the tutorial on YouTube, I recommend that you instead use the instructional resources here, which have been vetted as being of high quality.
Specifically, the sample chapters from Mark Christiansen's books give excellent instructions for avoiding issues of the sort that you're facing.
The weirdest thing about Keylight is its implied superadjustability when, in practice, none of the adjustments are necessary for 95% of the keying work most of us do. A single click on the background process color and BAM, Keylight is like magic.
Assuming adequate lighting and video quality, begin again with Keylight, starting from scratch, no adjustments, Select the key mask color and then resist the temptation to tweak all that other stuff.
Open the view dropdown and examine the choices. The one that may make the most sense to you is the STATUS display as you make tweks to the controls. You need to pay attnetion to your selection at the REPLACE METHOD, generally, you want Source.
Beyond that, Keylight is all experimentation.
I will add that most of us who have come up through many years of keying in AE with inadequate video sources like 3/4" or DV have learned to stack muliple copies of source footage and apply different filters and masks to the various layers to arrive at the perfect matte.
Agree with previous posts.
Keying requires a bit of fiddling and adjusting of the parameters.
For your issue..my guess is you need to adjust the 'screen gain'
Well, just copying settings from tutorials won't do anything for you. Keying is an art that takes practice to make it perfect. Even minor variations e.g. from uneven lighting can hugely influence the result from shot to shot, requiring different settings every time. and of course David is right - for all intents and purposes, on the best of days Keylight is being a ***** in its own right....
Thank you very much for your help.