ControlTips are always temperamental, in my experience, frequently taking way longer than is reasonable to show up, or, as in your case, not showing up at all. Since my forms are mainly "full scree" I go to the trouble use a short label, the width of the form, across the bottom to provide information to the user when a field is entered.
I've seen your question posted many times, and the only successful resolution I've seen posted was a case where the developer always designed his forms, and after he had all his controls in place, functioning properly and with the ControlTips working properly, overlaid rectangle controls to "group" the controls logically. When done, none of the previously working ControlTips functioned. The resolution in this case was to select each rectangle, goto Format on the menu and select "Send to Back." That done, the ControlTips worked again.
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