comcast users unable to see site on comcast business

I have a couple of users who work from home, and are having random outages, though the outages only affect access to corporate email and sites hosted on our comcast business connection. 

I've worked with them trying to troubleshoot the issue by doing the following:

-packet sniffer on the firewall watching for their IP (it shows up after a while (typically a few hours), but the issue has cleared by then)

-from the main site inbound and outbound traffic all appears to work. 

-attempting to ping or traceroute to them fails, but anyone else outside works fine, both locations can ping or traceroute to anything but each other (until the issue clears)

every time this happens, it only seems to be affecting users in the one specific area on comcast. users from any other provider have no issues.

this also seems to occur multiple times each week. 

to me this sounds like a comcast related routing issue