The following are some causes for wilting in poinsettias
Wilting/Epinasty in Poinsettia
The most common cause of wilting is a lack of water, and application of water should correct the problem. If plants remain wilted, the root systems should be inspected to determine if root injury has occurred because of excess soluble salts, root rot pathogens, or fungus gnat larvae. Growing medium testing laboratories and plant disease and insect clinics should be utilized for proper identification of the reasons for the wilting.
Flowering plants which have been sleeved and shipped to the retail outlet might show symptoms of wilting when sleeves are removed and the plants have been in the retail outlet for a couple of hours. The first response is for the florist or store personnel to apply water, but moisture might not be lacking. This wilting is referred to as epinasty. Varieties differ greatly in their susceptibility to wilting. Mechanical injury or bending of leaves during sleeving can increase the amount of ethylene being produced by the plant. Increased levels of ethylene can lead to droopy plants. Length of time in the sleeves also has an impact on the extent of the epinasty. Growers should avoid rough handling of the plants during sleeving and the amount of time that the plants stay in sleeves should be minimized.
Information from the Poinsettia Problem Diagnostic Key on Physiological Disorders from North Carolina State University http://www.ces.ncsu.edu/depts/hort/poinsettia/corrective/a11.html