Sometimes it happens that the jQuery datepicker is close to another active element, e.g. a slideshow, and when it pops up it remains on a lower layer, partially or completly invisibile.

I read several complicated js workaround about that, but the best solution is simpler than expected and can be found browsing the dom.

.ui-datepicker{z-index: 99 !important};

NB: the number 99 is only a direction choosen to be fairly sure that it works.