In addition to reducing oxides the flux acts as a surfactant, promoting wetting of the solder across the target metal. 
Hand soldering can provide a mechanical assist to promote the spreading of solder.  In addition, the soldering pencil is hotter than the air in reflow. So, the same flux can give different results reflow, versus hand soldering. 
I like Dave's input, and agree with Bob. While most WS fluxes are more aggressive than RMA they may not promote wetting as well like good old RMA. 
You need something that sticks around after the oxides are gone, not something that dries out or burns off. 

Guy 


On Thu, Oct 29, 2015 at 2:32 AM, Deng RongJun <[log in to unmask]> wrote:

> Hi Technetters
>
> I have got a problem with a SMT type mini USB connector. The four pins 
> contacted shield are no-wetting after reflow soldering.  But can be 
> solder well by iron.
> The pins only plated by Nickel, is it possible the oxidation film 
> creating on pin surface and then affect soldering?
>
> Picture:
> https://goo.gl/photos/qap1CBEWzUEJEc5i6
>
> Thanks in advance.
>
> RongJun Deng
>
>
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