There are a few ways to raise Brix in plants, as mentioned anything that helps your plant bring up nutrients from the soil, will help.
Potassium is important for brix, it is possible it is leaching out of your soil due to all the rain, so you may need to add more.
another way to get nutrients to your plant is by foliar spraying, seaweed and/or yeast extracts have been shown to improve quality of fruit. of course you want to apply this when you know it wont rain for a day or two, spray in the early morning or in the evening, when its cooler.