Do you have too little flux on the solder joint but too much on the circuit board?
Always check, even when fluxing:
Often the devil is in the details when it comes to precision fluxing. If you can adjust the flux application that applies the flux to the board, you can control the result quite accurately, even if the flux is applied quite quickly. New smartphones, in combination with a flashlight, are able to record videos at 60fps (frames per second). When a flashlight is used to illuminate the flux stream, you can detect it quite easily and see where it is applied.
In order to keep the assembly clean, you can also hold a thermal paper or sandwich paper in between. This paper clearly shows the direction of the flux stream. A small glass panel can also be used.
The positions can then be optimized and the performance parameters of the dispenser should be adjusted. This is done via the software of the machines. The ratio of speed, time and power of the dispenser is important. The simplest way to control the flux is the automatic module for flux stream control, which under certain circumstances can also be retrofitted in an Eutect systems.