Sounds like he might just be malnourished. This, unfortunately, is common with breeders who take on many species at once. Often, slower animals aren't fed because the breeder will neglect to feed them individually, and the result is malnourished frogs. The fastest and largest will ultimately prevail. This is sometimes concidered a method of culling as well.
Most breeders won't bite more than they are willing to chew. Some will. Could also have been the dealer, if purchased from a pet store or jobber at a market. We'll skip the problem and head to the solution though 
Make sure the temp is in the middle of the ideal range. Keep it at about 82 degrees farenheit and keep the humidity at about 60%-70%. This is high, so don't always keep it there, but while your trying to get the frog to stay warm and hydrated it should be up there. Make sure there is clean water available at all times, and avoid bright light during the day and cover the enclosure to avoid any light at night if he's kept in a room with any more light than an alarm clock. Get some dull foreceps, or use your clean fingers with no soap residue on them to hand feed some the fattest dusted crickets. Make sure your feeding the crickets on a healthy high vitamin diet 12-24 hours prior to feeding. Some fluker's cricket food and some kale is sufficient. If he will take the crickets by hand, use mealworms or silkworms tomorrow. Only use these for a short time, this is to put some meat on his bones.