Armando,
Is the problem in the same spot on the board with every run?
If so, there might be something about that particular location that is part of the cause (ie. ground or power plane connection).
It sounds like you're seeing blow holes, or insufficient fillet on the top side of the board. If this is true: 1. Verify against IPC that the assemblies fail to meet spec...nothing like chasing a process problem down when you don't need to! 2. Verify the consistent application of flux through the machine. 3. Try applying some flux to the top side of the board, as well as whatever flux you're applying to the bottom side.
Blow holes are caused by releasing gas. Ensure that there is room for the solder to outgas around the componet (sometimes, components mounted too tightly to the board can prevent efficient outgassing, causing blow holes).
The insuficient flow/fillet could be caused by: 1. Contaminated components (flux should correct this) 2. Insufficient dwell time (though I'd expect to see that throughout the board). 3. Insufficient heating (which could be caused by wave temperature variation, or a difference in thermal characteristics at that part of the circuit (ie. large copper pour/ground plane, no thermal relief vias, etc).
Cheers, ..rob
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