you are seeing a typical issue that is caused when such small volumes of paste are printed....there are several reasons for it..the tiny amount of flux in the small deposit becomes exhausted much quicker then it does in a larger deposit ( the flux may bleed away from the powder when in the reflow oven). If you have no fluxing action left when the solder is above its melt temp then the solder spheres will not flow together. The flux is the medium that transfers the reflow oven heat to the paste so if there is little flux and it is drying out fast then the solder powder doesn't get the heat transferred to it efficiently. Trying a paste with more flux may help reflow but will give issues with printing. Ask your paste supplier for a paste with a thermally stable flux that is proven to reflow ultra small deposits.
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