Hi Sara!
It may be the skins starting to sag, but I’m wondering what style and size koma you use and where you are putting it on the skin. I’ve found that if I place the koma too far away from the neo, the shamisen can sound kind of dull and lifeless. The rule my teacher taught me (and many teachers teach) is two finger widths away from the neo. You can try moving that even closer to the neo as well. It should help to sharpen things up. If that doesnt help, it might be worth it for you to invest in a higher ivory (zouge) koma, since ivory will help sharpen the tone a bit. Its a lot cheaper than replacing the skins, that’s for sure!
On another note, make sure that your shamisen is not too close to your body. For me, 1/2 way between my torso and my knee is the most comfortable. If you place it too close to your body, you lose a lot of resonance.
One other thing you can do is strike at the top of the bachikawa. If you strike where the wood edge is, you will get a sharp clear sound. Be careful if you are using a plastic bachi though!
I really hope one or more of these tips help!
Do you store your dou in a washi bag?