If it jerks forward when you shift into first, it is not disengageing. If it fails to move after you put it in gear, it is not ENGAGEING.
If it has been a while since you rode the plates may be stuck. If it's now COLD where you live the oil may be too thick OR you should warm it up more. The cylinder should be warm almost hot to bare hand especially when it's Kold out. Also, while it's warming up work the clutch lever in and out, many many times if you have to.