What do you mean by "half-full"? My tank pressure gauge needle is just over half way around the gauge at a reading of 62 bar. This is the normal reading when full and during use. In a CO2 tank, as soon as the tank pressure starts to drop, that means your tank is almost empty (or at least that there is no more liquid CO2 in the tank and you're running on "fumes").

There is a problem if you're blowing through a 5lb tank in a week. Are you sure they are filling it full? One way to check is to weigh the tank before and after the filling...

Otherwise there is definately a leak. Maybe where the regulator connects to the tank valve? You are using it upright and not laying it down?