You may need to delete the Shipping Rules you currently have and create new ones. When you make new Shipping Rules to replace them, make sure they don't have overlapping thresholds for Min/Max Item Total. You should have one Rule with Min. Item Total = 0.00 and Max. Item Total = 20.99 (or whatever max. you want for the low rate), and another Rule with Min. Item Total = 21.00 (.01 more than the other Rule's max.) and Max. Item Total = 0.00 (that's treated the same as a blank setting, i.e. "no limit").