Soldering copper to brass (shower / tub valve) lower valve fitting "upside down". Will not take solder. The brass, to the eye looks like it is not taking heat. Rub the solder against the fitting does nothing but will melt on the copper. Used plenty of flux. Using "map//pro torch. Let flame sit against fitting 2 1/2 to 3 minutes, nothing. What now ? Jim

Are you quite sure that the fittings are dry with no moisture and that you have cleaned and fluxed both fittings? Did you apply heat to the larger fitting. For a step by step video click on;YouTube - Easy Soldering Brass to Copper
Good luck, Tom

As speedball said, make sure valve and pipe are both cleaned very well, fluxed and make sure there is no water at all in the pipe/valve. If you have held a torch to it for three minutes you have cooked the flux out and will need to reclean and reflux pipe and valve. Good luck and let us know how it goes.