Answer: I have a feeling you just need to give them more time. The dates to maturity rating on seed packages can be a little deceiving - -the number of days usually refers to the days between planting the transplants into the garden to the time you begin harvesting full-sized peppers. It can take several more weeks for peppers to color up to their "mature" color. I've had the same problem with red peppers in my northern Vermont garden -- lots of green, but few red before the frost hits. As the weather begins cooling and the days grow shorter at the end of the summer, it really slows things down.
I wonder if you picked most of the green peppers, and left just a few on to ripen, whether that would speed things up...though I know you grow yellow peppers to have yellow ones, not green!
Another trick might be to use black plastic around the base of your plants. I do this to help warm up the soil in the spring, but I'm sure it helps too as the days grow shorter.
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