There needs to be just enough space between the parts for the solder to wick in - too tight a press fit and it wont go all the way through. For parts laid on top of each other, one trick is to make a couple small prick-punch marks on one surface to hold them apart just a tad. When the solder goes liquid and flows, you should be able to see it on the far side, if not adding a little more solder is fine (too much and it will blob). Best be is to heat it on the side away from where the solder is, that way the metal melts the solder, not the flame, and it flows the best. If the parts are different thicknesses, heat more on the thicker one, so they both get up to temperature evenly. Part science, part technique, part art, lots of practise, very useful skill!