What's probably happening is that you're using too much water or mixing your colors directly on the palette without any organized method. With acrylics, the key is to start with very little pigment and add color gradually, because they dry fast and lose intensity if you stir them for too long. The amount of water matters too: if you add too much, the colors end up transparent and dull.
One thing that helps is mixing your two base tones first, and then if you need to adjust the brightness or darkness, do it with a third color. Those videos where everything comes out "perfect" - almost always it's because those artists already know exactly how much water and pigment they need. It's experience more than magic. Try working with smaller palettes, do test runs on separate paper before putting it on your actual piece, and don't stir any more than necessary.