Without having more detail it is difficult to diagnose the actual issue. With cracked or failed solder joints you would want to do a few things to determine the actual root cause. Is this a sudden issue?
In other words can it be related to a specific batch of components and/or PCBs? Take a look at the fracture surfaces, what do you see? Does it appear that the solder joint is just "pulling out" of the solder fillet (sometimes called "foot-in-mud"). If so it could be a plating issue on the components.
Is it fracturing at the pad surface? Could it be related to the metalization on the PCB? Those are some concerns to look into.
Since you are suspecting reflow issues, perform a profile, being sure to place a thermocouple adjacent to the solder joint in question. If this is a densely populated PCB, with components of different sizes you want to make sure you profile in multiple locations to make sure you have the proper reflow time and temperatures at each location.