All posts beginning with “I” not shown in frontend

Hi there!
I l-o-v-e this Plugin.
First time ever having trouble:

If I defined a certain category to be shown, it has 6 post, 6 posts show in backend, but only 5 posts show in frontend. Same if I list the whole mothercategory with more posts –?still this one post will be missing.
The post is titled “Isoniazid”

If I rename the posts title to for example “F Isoniazid”, all the sudden, the post is shown in back and frontend.
If I rename it back –?not shown any more.
If I name it “I Isoniazid” – does not show.
Name it “Iso-niazid” –?not shown.

Actually all posts beginning with “I” are not shown in their catlists, frontend-wise.
But you will be able to search for them and thus open them etc.

Can you help?