In the words of Oliver Twist "please sir, may I have some more?"
Aponogetons need heaps of food otherwise they use up the nutrients in their bulb and then suddenly die.
They grow really well for a few months after you get them. This is due to the nutrients/ reserves they have in their bulbs. Once they have used that up, they look for food in the soil and water. If there isn't enough, the plant dies.
If the plant was getting more light before it went into the tank, that would also knock it back. Give it lots of light and lots of fertiliser.