Usually for questions like this, posting a packaged workbook is incredibly helpful, otherwise we are guessing as to what is going on on the marks cards, what the data looks like, etc. However, playing around with superstore, it looks like the issue is the zero height gantt bars.
if they are out in the "open" there is a buffer around them that will show the tool tip:
however, if there are overlapping marks, this buffer goes away, and since the gantt bar has NULL height, there is nothing to hover over (I am just guessing that this is the cause)
if you add a value to "Size" (500 in this case) the gantt bar can be hovered over, and the tooltip shows:
based on your screenshot, I am guessing that you don't actually need a gantt bar, but instead want a line. I would recommend creating a "line" custom shape , and use that instead.
Hope this helps!
I will try the create a "line" custom shape. Every type of bar should have the ability to display a tooltip in my opinion. Another one of my assumptions shattered...LOL
The point of a Gantt bar is to set both the starting point and the length of the bar rather than it being forced to start at the axis, when used in this manner, it displays a tool tip as expected. In your case (unless I am missing something and you have a measure on size), you are attempting to use the Gantt mark type as a shape mark, and while sometimes this is useful (say you wanted the line (Gantt mark) to extend wider than your bar marks, you could adjust the sizes of the bars and Gantt marks to achieve this), a Gantt mark will not behave exactly like a shape mark. In my opinion this is really an artifact of Tableau's flexibility, rather than an issue with the Gantt mark type. Since a Gantt bar requires a starting point and a length (or a start and end point), it would be completely reasonably for Tableau to render nothing until a measure was added to "size"; however, by not forcing this (and other similar) requirements, Tableau allows us more flexibility to create vizzes that don't fit in to the out-of-the-box mark types (Sankey, waffle, radar, etc). Tableau is definitely different than other BI tools, and there is a learning curve, but as you start understand how it handles data, and how it generates visualizations, you can leverage those differences to tell better stories with your data.
here is the example I mentioned with the Gantt Bar (nothing on size) wider than the other bars, hovering at the edge of the Gantt mark shows the tool tip:
I went ahead an threw a line shape together. I use a program called paint.net for creating shapes:
the line shape is attached, 32px wide. 2px thickness (png is 10px high, but transparent)
Hope this helps! Happy Tableauing
EDIT: typo, better screen shot of line shape with tooltip, modified link for paint.net
line.png 242 bytes