<human>: Can you condense the text into a few key points?

So I have been dating this girl for a few months and mostly everything is great.  However, we are having some issues sexually, specifically she does not like giving head.

She has given me head twice, once to completion.  Afterwards she started saying how "it's so hard giving head" and that she "can't breathe very well" and "it sucks"(no pun intended).  

I didn't really say anything after but since then she has not even tried to go down on me since then.  I will give subtle hints to her that I want her to but she obviously does not want to.  

On the other hand, I am going down on her and she likes it(she has told me)so it's not like I am not holding up my end of the bargain sort of speak.  

How do I go about telling her that this is something that is important to me without it sounding like some sort of ultimatum?  I feel like this may be a deal breaker but I want to communicate this delicately so we can work through whatever issues she has with it.
<bot>: My girlfriend does not like giving head and it is something that is important to me.  What's a good way of delicately communicating this to her?