I have one of these, I also bought it secondhand and my steam pressure is such that I texture milk for two cups in 15-20 seconds.
I would say you have a restriction in your steam line. That is the nozzle, steam arm, steam valve, or the copper tube from the valve to the tank. I would pull bits out and clean or descale them as necessary. If you are half handy you might be able to do it yourself. Its all very straight forward.