Sky boxes are notorious for not providing enough voltage/signal along the HDMI lead to power a switch. They only 'just' push out enough to meet the HDMI standard, whereas most other bits of kit exceed the standard and can power a switch. The cable length is not an issue.
It is possible that a 'powered' switch (ie with a mains supply of its own) might work - but make sure you buy it from somewhere that takes returns, in case it doesn't.