The most obvious cause of the non reflow of paste is that the temperature of the reflow process has been incorrectly specified. This may be the first board of the shift before the oven has reached reflow temperature or the incorrect paste is being used. It may be the oven is used for dual functions like curing adhesive and operators have not changed the process settings for reflow.

The same problem can occur if a partly assembled board is left exposed to the environment over a couple of days before reflow. The temperature for reflow may have been reached but there is no activity left in the flux to allow reflow and wetting to occur.