Presuming that you've done a "First time/Factory default" reinstallation rather than just a simple retune, then yes, cable length can make a difference. It depends on the quality of the cable. It can also be a case that one or more cables isn't terminated correctly or has a break in the outer sheath so that water has got inside. There could also be a difference in the comparative sensitivity of the TV tuners that's contributing to the issue.
Do the simple stuff first. Check the way the plugs are terminated. Make sure there's no little filaments of the braid shield touching the central core. Look for kinks and sharp bends in the coax cable.
For a cable length of 15ft to make a significant difference the cable would have to be quite poor - e.g. the sort of quality you get in those DIY TV extension kits from B&Q/Maplin/Argos - and the signal itself would have to be quite weak. The combination of the two things would be enough to make some of the weaker channel groups start to break up.
In many cases the underlying reason for the signal break-up is not signal level but signal
quality. This is far more important than signal level.
If your TV has a way of showing signal strength and signal quality then you might want to check it on the channels that are causing the issue. It's entirely possible to have lots of signal power (80%+) but have very poor quality of signal. Boosters and amplifiers won't help this. The
quality comes from the aerial (correct spelling
). The rest of the system should then be built to maintain that quality.
Simplifying your system might help diagnose where the fault lies. It's one of the things we do in the trade when troubleshooting. Try connecting the feed direct from the aerial down-lead straight to the living room TV. Check the picture on the same channels that have been causing a problem upstairs. If all is okay, swap the connection and route the aerial signal up to the bedroom TV. Check the picture up there. If you're still getting a problem then think about changing the cable for
WF100 and some decent aerial plugs.
Beyond the above then you could do with calling an aerial installer. They'll be able to check the signal margin and then fit the correct gear so there's decent quality signal to all TVs.