My starter has matured and become less active

If you're sure it's still alive, but it seems less active, maybe you should try giving it less to eat. You can either lower the ratio at your normal feeding time, or feed it less often. If you give it more food than the yeasts and LABs can eat, it will dilute the concentration of yeast and LABs in the culture, because they are swimming in food. They should ideally peak out before being fed again. If you have some flexibility in your schedule, try not feeding it until it is at its peak rise, and starting to drop. Another thing you can do, if you just want to jump start its production rate, is put it in a warmer spot for a while. If your oven has a light in it, you can put your starter there and leave the light on. A pilot light in a gas oven will work too. Check to make sure it doesn't get TOO hot, of course. At the warmer temp, the yeasts and LABs get a lot more active, so you will have to watch it and feed it appropriately.

I'm in the same camp as DavidEF, has your weather begun to change?  I know mine has, and my starter has needed a different feeding regime to go with it.