There are many reasons that can cause a dog's tail to be limp, or held down. Some causes may be related to:
Overexertion
If you take your dog for a long walk or jog after being inactive over an extended period of time, your dog may show signs of a muscle injury. The tail may be flaccid and loose without the ability to wag. These signss are often accompanied with swelling at the base of the tail, and can be quite painful.
Exposure to Cold Weather or Water
If your dog is sensitive to temperatures, being exposed to cold weather or even cold water can trigger a limpness to the tail. Some dogs are much more likely to have this occur, such as Retrievers who seem particularly prone to showing signs.
Prolonged Confinement
Keeping your dog in a crate that is too small for him over a long period of time may cause tail injuries This is due to the tail being in one position much too long and is similar to the way our limbs can fall asleep after being in one position for an extended amount of time.
Muscle Injury
In some cases, tail injuries can be caused by a sprain. A sprain is categorized as an injury to a joint and the area surrounding it. An acute inflammation may occur due to irritation of the muscle and the tail goes limp. After a few days, the tail may slowly recover to the point of being normal once again.
Medical Issues
Conditions such as prostate trouble, anal gland inflammation and osteoarthritis may cause your pet to hold his tail down to counteract pain and discomfort. Any dog can be affected by a limp tail, although dogs with longer tails tend to be affected more often.