Hello
There are several possible reasons for your situation.
What brand and model of board do you have? Have you installed the drivers?
Look at other software to see if there is any pressure sensing (e.g. in Photoshop). If there is pressure in other software, then it may be a problem with the settings of the sai software or the brush settings
Please check
1. Whether the brush has been checked Brush Pressure Control Brush Diameter, and whether the minimum diameter of the brush has been set to 0
Execute the Other - Options in sai. Other--Options, switch to the number pad tab, and see if the number pad option setting here is the cause of the problem.
3.There may also be problems with the SAI software itself, try reinstalling it or switching to a different SAI software. I'll give you the SAI you're using, at the bottom of this answer, click on download to access the download.
4. If reinstalling and changing the software still doesn't work, then you can try reinstalling the driver for the digital board, plugging the board in, and consulting with the board's official customer service to find a solution if necessary.
You try it first, I hope it works for you