A good picture of an example solder joint might help.
Things that tend to do that include use of a rosin core flux without cleaning it off. More active flux that eats more metal, or the part leads are dirtier and/or more oxidized than others.
Other causes could be overly hot soldering iron or heating the joint too long so it cooks away the flux, or a dirty iron tip that leaves deposits in it, or using too much solder and thus too much flux, or a solder with an especially high flux %.
On the other hand it could just be that you were used to using no-clean flux, or used to seeing finished products where they had cleaned the flux off, and/or it was a water based flux so all you see left behind is some hazy whitish film instead of dark brownish flux.
Originally Posted by dd051 /img/forum/go_quote.gif So far, the dark and fluxed joints have been only PCB related.
The solder I am using is a 63/37 (.020 diameter) blend .
I did not flux the pads before soldering.
I was trying a 1.8 mm chisel tip and a cone tip with an iron set around 600 F.
I was also wiping the tip between joints so it appeared nice any shiny silver/tin colored.
Also, I tried some more soldering a bit earlier and nearly all of the solder was going onto the tip, (top and bottom of a chisel), not the pad.
solder not sticking where it ought to is a sign of not enough heat, need of flux, or both.
As usual i recommend about 700f for leaded solder (750 for unleaded) and can't speak highly enough of the Kester 186 Flux Pen - which is like a felt-tip marker for applying tiny amounts of really good flux.
In addition to what ericj wrote, I'm wondering about the soldering technique, if the solder is being applied opposite the iron on the joint between the two parts. I only mention this because I have seen people who try to solder by applying the solder between the joint of the iron and one of the parts instead. Either way, some flux residue is normal but you might be using too much solder because of the problems you're having getting it to flow. Try a test solder joint by heating up the area a moment longer, with the iron at the higher 700F temp ericj suggested. If you are soldering in a part that is very old and the leads are a bit contaminated, if the part is not ESD sensitive consider wiping the lead with a dry paper towel as the abrasion will clean it some. If the part is ESD sensitive more flux and/or a more aggressive flux should be used instead.