There can be different reasons for solder not taking to one side of a fitting.
One reason can be dirt, as has been said, but you shouldn't really have dirt with end feed fittings, like Yorkshire have, coming often from around the solder ring area. I clean my end feed fittings with steel wool, so they have no dirt.
Another reason can be overheating and/or burning the flux away and tarnishing the copper so solder won't stick. Solution is to not overheat if possible and also apply flux dabbed on end of solder as you do the joint. (If you do want the resolder a tarnished bit, clean it with emery cloth and apply flux to redo)
Stigster got my last reason, - if a pipe is held slightly out of line with a fitting, the the solder will not have good capillary action at the widest point at bottom of joint. This is why I am sceptical about clipping pipes to walls that can be all shapes. Also tends to happen with slack fittings that some cheaper brands are.
If you are doubtful about a joint, heat it gently and pull it apart to look at how well both surfaces are soldered