Start by calling and requesting the $20 credit for missing an appointment, in accordance with the customer service commitment. https://www.xfinity.com/support/account/comcast-customer-guarantee/
Then, when you have their attention, insist that they reschedule the missed appointment ASAP.
Hi, mharness. Sorry we missed you. I can help get you rescheduled for a technician appointment. Please send me a private message with your full name, address, the phone number associated with your account, and your availability so I can assist. To send me a private message click my name, ComcastMax, and then click Private Message Me.