Arranging pre-heat is almost a must.
I have done similar, where I have even used two torches to achieve the result needed.
So, either a second torch (my preference) or a bigger torch (easy to overheat), make sure both parts are clean and use flux.
I also made use of the kitchen oven (asked first though) but, one has to work quick as the heat goes out the open door, also working conditions can be cramped...