Did you look at the control in design view? There is an error flag. When you click on it you'll see that Access is telling you that there is no such field as Available in the record source. Yet currently in the xtab query there is such a field**. I would never think to base a form on a xtab query because one time, a field is in the query, the next time it's not. So I can only imagine that if it was ever there, the link has been lost.
xtab queries are sometimes used for reports, but you have to declare all the fields as parameters in order to avoid the volatility of the xtab query. You'd need to explain why you think you need to base a form on a xtab query. Better yet, maybe explain what the goal is without saying what doesn't work.
EDIT
**Sorry, mis-spoke. In the sample db, there is no such field but I was presuming it was there before and that you were able to select that field from the property sheet at some point. Just for fun I tried declaring the fields in the query property sheet (Column Headings) and it seems to work. Not something that I ever envision doing for myself though.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.