After several weeks of intense oral training Sissy should be able to swallow just about any size cock you put in front of her!!! So it's understandable that Mistress might lose patience and have to....force the issue!
Although it may seem too much sometimes a little tough love is what Sissy needs to break through to the next level!!!!
It's for her own good after all!!!!
Make sure that Sissy says "Thank you!" when you're done!!!!!