Do you see something to the effect of SMCD3GMV ? If so, there have been many posts here and at other forums about the weakness of that device's wireless radio signal. We have been recommending to posters to have the device placed in to "bridge" mode by Comcast so that it acts as a straight modem only and to use their own separate high quality wireless router with it.
Didn't see your response. The above number is probably it.
You could try accessing it and changing the wireless channel that it operates on but that hasn't seemed to help others. You could also request a swap to an Arris D3 capable eMTA (if you are using their phone service) and use your own router.
Read EG's post above, if you are using an SMCD3GNV, this device is notorious for its poor wireless range and performance. The best solution in this instance is to call Comcast at 800-363-2416, have it placed in bridge mode, and use your own high quality wireless router.
If you are some other device or a different situation, then you need to explain more fully what you have, how things are setup, where things are setup, etc. Wireless range is affected by many factors, like devce interaction, signal strength, wall/floor constrution, proximity to other devices, wireless interference, etc.