Please check whether one or both situation below is occurring:
Incorrect alignment between the tool and the tube;
Tube wall thickness different from the recommended for the SPIN Flaring tools.
The Flaring Spin tools are designed to work with copper tubing 1/4’’, 3/8”” and 1/2’’ with wall thickness of up to 0.8mm, and up to 1.0mm for the 5/8’’ tubing diameters. Using the Flaring Spin tools on tubing with thicker walls than recommended may cause undesirable deformation and assembling issues. Check our Documentation for details.
Also, it may happen that the nut is not standard. Please refer to the documentation in the link for further information, and the video below, for the cases where the nominal wall thickness is 1mm and a slight swelling next to the flare is occurring.