Not Getting Advertised Upload Speeds

I have a 400 download / 150 upload plan and while I'm getting at or near the advertised download speeds on speed tests I'm maxing out around 20 mbps upload speeds. Using the X6 gateway in bridge mode to my own router. The xfinity assistant immediately defaulted to sending out a tech instead of going through any troubleshooting steps, but I'd like to at least know what the issue is before setting time aside to schedule an appointment.