How are you testing the speeds? Make sure the device you’re testing on is a gigabit device. 

In regards to your upload, you’re not going to see 100mbps upload on it. Comcast only provisions their xfi devices to higher upload speeds. Its only lately they’ve been upgrading the higher end modems to faster uploads. 

https://www.reddit.com/r/Comcast_Xfinity/wiki/knowledgebase/next-generation-internet/

I had a similar issue quite a while back. The Arris Tech has me go into the operation tab in the admin portal  (instead of the configuration tab which is default on log-in).  I went to the status tab and I had a number of the 24 downstream “channels” not working. In addition, the signal to noise ratio (SNR) was way off; everything was in the 10-16 dB range instead of the expected 34-38 dB range. The tech told me to contact Comcast and insist that a tier 3 ticket be crated to go back into their server and rebuild my account. I did some internet searches to become conversant on the issues. As the Arris tech predicted, the Comcast tier 2 tech was not happy … but after asking to speak to a supervisor, a ticket was created. Within 36 hours, the tier 3 tech had completed the rebuild of my account and my download speeds increased ten fold. I periodically go back in to my Arris device to check.