It's not a good idea to RE- solder joints, even if your well practised.
I would buy new fittings, clean all ends & fittings and re-do, try not to use excessive heat & get plenty of solder into the joint.
:) hi elle, I take you are using flux on your joints, as without it it is hard to get a good seal as the solder won't run around the joint properly. ( that's what I have found anyway )
mungo