Soldering is quite a bit like cooking a gourmet dinner: there are raw materials, a process for putting them together, a heating temperature and time, and finally a test to verify things went as well as planned.
I’ve become used to the fact that people tend to blame material suppliers for problems with finished products. No material is perfect – but if they are used in the recommended way, soldering products are very consistent. Make sure you double check your recipe, because even the best ingredients can’t cook themselves!