I agree with what above girls said about this.Tricking your gf just for that is really bad.In a relationship, sex or those things is not everything ( in my opinion) as long as you love someone so much.
No one can force girls to give you head.If she wants to do it, she will....But in fact, I feel a bit scared of u when you use such language.My fiancee never talks to me in that way.It sounds rude. I guess if you give her more respect and talk to her more tenderly,maybe it will work.But anyway, it really doesnt matter if she gives u head.You should find another one if this is so important to you