One possible explanation is signal loose. When a cable wire is split using a splitter its signal strength is cut in half. If you cable line comes into your house and then goes through a splitter, you just cut your signal power by 50%. Then ones of those lines runs up to a room that has a TV and your cable modem and your have another splitter, you just cut that signal by another 50%.

Solution: You need to have a cable run from the entre-point into your house directly to your cable modem so that there is only one split. This will maximize the signal strength going to your modem. Unlike your TV which gives a picture even when the line is split several times, this is not the case with a cable modem.

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