failure to thrive

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A significant delay in the physical growth of an infant or young child, caused by an organic, psychological, or environmental disturbance.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Used other than as an idiom: see failure,‎ thrive.
• noun uncountable Poor physical growth or level of weight, relative to one's age peers.
• noun uncountable, medicine Abnormal weight loss, malaise.
• noun countable, medicine Instances of poor physical growth or of abnormal weight loss.