Simply wanting a power won't give it to him. He didn't get anything from Jean. Perhaps more thought on the nature of his powers is called for question?
Oh, I know that. The situations the MC has been in have caused emotional distress or have been outright deadly. It seems he has to feel that a power would help him in a particular circumstance. He also may need a certain potential to obtain the power as well since the powers are:
A. The true power form of the person he copies from (Rogue's power actually being more of a death field she should be able to turn off if she stops inhibiting it with her anxieties). Since Jean's power is something else entirely, and he has not thought of getting that power, he simply has not even tried is what I really mean. So far all we know of Null's power is that he feels a potential danger or stress and thinks about overcoming it in some way and he , "Grows new powers out his @$$." as Spyke indicated. But he directs it by thinking it would be useful to have that power.
B. Weaker than the original it seems. He may also have a limited number of powers he can copy at a time and may need to juggle a limited number around according to what is useful to him at a particular time.
LOL. it is fairly easy. First encounter with Jean and she is like, "Your good at this." Score card A and two orgasms. Alas, I was not good enough to get her to go all the way. Gotta get her love and trust up more. At least she expressed guilt that she did not help me out.
On another note, I did encounter a duplication bug in the inventory. During the end of Winter mall encounter I bought the green thong for Rogue. On giving it to her after recovering, I still have it in inventory and Rogue has the option to use it as well.