well i use Sai and Photoshop and i know that sometimes those can have issues with the pen pressure not working at first but they can usually be solved easily by goggling the issue!
also! because this happened to me recently and i couldn’t figure it out for like 3 days hurhur, in Sai there is a bar for the “min size” of brush you are using. if that is set to 100% then pen pressure wont work because its always drawing the line as large as possible. i dont know how that got set to 100% but it took me an embarrassingly long time notice that was what was wrong OTL
ugh 