You need a signal booster/amplifier like the ones listed here (up to 8 ports):
http://www.amazon.com/Motorola-48409.../dp/B000066E6Y
That's assuming the cable company checked the signal strength in the main cable line to your house and it's within specs and the cable service center/office boosted the signal from their end.
I actually have 2 lines coming in, one for internet and one for cable tv, but it all depends on the installer or what you request in your order....the price is the same per month, so it's not like it's going to cost you more if you have another line.