Start with the most common culprit: the grind. If your grind is too coarse, water rushes through, and you get under-extraction—sour, weak aroma. If it's too fine, you get over-extraction—bitter, ashy, and oddly flat. Your goal: a 25–30 second shot. If your shot pulls in 15 seconds, grind finer. If it takes 45 seconds, grind coarser. Also, check your beans. If they were roasted more than a month ago, they've lost most of their volatile aromatics. That's not a fix; that's a re-buy.