You stated that you dont know the names of the plant, Is it possible to get some pics of the plants in question. Or go here http://www.plantgeek.net/plantguide.php
and see if you can id them.
It is possible( very likely, for the uninitiated,lol,) that you have been sold non aquatic plants.These plant will appear to do well at first then start to rot and die because the are not true aquatic plants.
HTH
The other situation is that mentioned by the others above,Submersed and emersed growth.