After the latest updates, I no longer have pressure sensitivity in Flash. Also, I never tried it in Illustrator before now, but it doesn't work there either. However, it does still work in Photoshop.
I did have pressure sensitivity in Flash before. The pressure button is available and it's turned on, but there is no noticeable change in the brush tool.
I'm running Windows 8.1 and using a Huion H58L tablet.
I would try reinstalling your tablet drivers. Hope that does the trick. I use Wacom tablets and sometimes I need to do a reinstall after an OS update or if I plug in more than 1 type of tablet model and have driver conflicts as a result.
Finally got around to trying that. It didn't work.
Had the same problem now flash only works when i duplicate the screens if I don't the brush draws straight lines out the canvas.