As a matter of interest, when you have trouble with your i vowel, does it sound bottled up and dry to you? If that's the case, then you may be jamming your larynx up with too much breath, rather than allowing the adjustment that's needed which then allows the mechanism to function properly.
I will write some more, if you let us all know the feeling you're having, but as I think John said, a singing lesson over the internet is hard at best!!
Take care,
Ian Voice wrecker to the stars
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