There could be numerous reason's for this,for example
have you got a clear line of site i,e dish not obscured by tree's,building's e.t.c
Sub-standard cable could have been used,
Dish maybe slightly mis-aligned,
But as it's been like it since installation, i suspect the dish is mis-aligned....it only has to be marginally out to cause low signal. I'd get sky back to check it out.
FWIW no you can't just add a readily available signal booster like what is used for terrestrial aerial's to improve signal.You can get Satellite specific in-line and powered amplifier's but not really intended/or needed for standard Sky installation's. As i said above get sky back!