It can be difficult to tell for sure when looking at the photo, but one of two things is probably happening. You haven’t let the under layer dry thoroughly and are lifting color rather than adding more. Or you have scrubbed too much and damaged the paper to the point where it can no longer accept color (which I suspect is the case in the painting above).
Wait for it! Let one layer dry COMPLETELY before adding more color. Wet-into-wet is a wonderful technique but takes more practice, so it isn’t the best way to get shadows unless you know how to use it.
Also, paint undergoes a color shift – watercolor becomes lighter as it dries. If you wait, you know your actual color, and have a better idea of what you need to do next.
You may be afraid of a anything approaching a hard edge. Often you don’t need as soft an edge as you think. Many artists believe that shadows should have three shades. Even if you aren’t totally satisfied with a dark-medium-light blend, it is a good way to start out. It gives you a plan and a definite stopping point. You can come back later and add more if you really don’t like it, but give yourself time to get over the need to fuss.
Your colors count too. It can get tricky, but if you have already painted the area, adding the complementary color can make a good neutral shadow. I recommend consulting a good color mixing chart if you aren’t sure of your complementarys. Also be aware that not all yellows, reds and blues are the same. If you use a warm with a cool you can get muddy colors. For instance, you shouldn’t use an orangeish-red with a bluish-green. An orangeish-red with a yellowish-green would work better. Ultramarine Blue and Burnt Sienna are a good mix for dark colors, so if you aren’t sure, you can usually use it.
Last but not least, consider your tools. Cheaper paper or a stiff brush will make it easier for you to damage the paper.
Now I just need to learn how to follow my own advice, lol!