If you post more detail about your tank - e.g. how long it's been set up, stock, filtration, water parameters etc. - we will be able to advise you better.
There are about half a dozen common problems which could cause the symptoms you describe, so more info. is definately needed.
In the meantime, it might be worth adding an airstone to the tank to make sure the oxygen levels are ok while we work on the problem.