Something similar developed on my bike around 8k miles. The procedure of pumping the lever as mentioned in a another post may work, but that means you have some air in the line so it is not a good solution long-term. I have bled brakes and clutches before, but this bike wouldn't clear the air no matter how what I did (Mityvac, hand bleed, etc.). The fix for me was to apply a bit of pressure on the lever while quickly cracking open and closing the banjo bolt up by the master cylinder. Be careful and lay out something to protect your bike because fluid will shoot out. That was the trapped air for me, but it may not be for you.
I haven't had to yet, but it sounds like you are due for a fluid replacement
When that time comes for me, I plan on changing the fluid from the bottom up. Meaning inject new fluid using a syringe with a tube hooked to it at the slave cylinder, and extract old fluid using a different syringe At the master cylinder until it is all new fluid. That will hopefully minimize the chances of introducing air by accident.