I was reflowing and a 2K 603 resistor partially tombstoned. There was solder connecting the lead to the pad, yet my DMM showed an open. When I heated both ends and the resistor came down and contacted the pad, the DMM read 2k ohms like it should. Why didn't the solder conduct between the pad and lead? Also, sometimes when I touch a probe to a solder blob on top of pin, it shows an open. Yet when I touch the pin above the solder joint, it shows conductivity. Is this the same case when there is a cold solder joint? The solder is touching the pin and pad, yet there is no conductivity. Isn't solder a metal and therefore should conduct? How come it doesn't?
Sounds like a classic cold solder joint. This happens when the solder and pad are insufficiently heated (time or temp), or the surface is not clean, and the wetting action does not occur properly.
If you use a meter to measure from one point to another on the solder itself, it should conduct, but between the solder and pad, there is not actually a good electrical connection, hence the open.
It's not that solder isn't conducting, but that it hasn't made a bond to the copper and therefore contaminants or flux are actually creating a non-conductive barrier.
As for the pin-to-pad conducting but not solder-to-pad, the pin is resting on top of the pad and likely making a physical connection (held in place by a brittle solder blob). The solder, however, may have a thin air gap left over from flux or contaminants, and isn't actually making contact with either.
It's amazing how easily solder (or other metals) can look clean but due to a thin layer of contaminant or oxidisation when you probe them you don't get an easy contact (you have to wiggle and press harder).
This is why sharp probe tips are so important. Flux cleaner/Ethanol/Cotton buds are good to have handy also.
What you're most likely experiencing is the flux or similar chemicals which are preventing you from properly touching the conducting solder. Using alcohol will help you remove them. The solder itself conducts (unless it doesn't make proper contact), it's just that your instrument isn't connecting to it directly as it should.