How do I get better at tasting?
You get better through concentration and lack of distractions. Just as you shouldn't eat while watching TV. You'll miss all the flavors.
Sit back. Take small samples. Let it lay or slosh around in your mouth for a while--several seconds at least. Think about what you taste and feel.
Soon you will find those one-tone flavors have a background to them or a subtle second or third flavor coming out of nowhere.
After a while, you'll start noticing those underlying flavors every time you use that ingredient and notice it missing when you have a not-so-good variety.