Hi there!
Water quality is most important to an axolotl's health. Have you tested the Ammonia, Nitrite, Nitrate, pH, and water temperature of the tank? Ammonia and Nitrite are the most important readings to take, as any reading higher than '0' in either of these is dangerously toxic to an axolotl, and can cause a great deal of stress to the animal.
Has the tank been cycled? If not, how frequently do you conduct partial water changes? And, how much water is replaced each time?
What size is the tank? Does the axolotl have any tank companions (other axolotls, fish, or other)?
What type of filter are you using?
Is there any water current going on in the tank? Axolotls prefer still water, and water movement of any kind (such as the flow issued from a power filter) can cause them a great deal of stress.
Impaction (ingested stones blocking the gut) can also cause an axolotl to float. Is there any gravel in the tank that the axolotl may have swallowed? What sort of substrate are you using? Is there any small ornaments such as marbles or anything small enough to fit in the axolotl's mouth that he or she may have swallowed?
Is the axolotl eating? What are you feeding the axolotls? How much, and how often do you feed them?