If you are sure your CO2 and lighting level is sufficient, then my suspicions willl be the way you plant it. Make sure the growth tip is not covered by gravel. What you can do is to tug the plant into the gravel during planting follow by lifting the plant up slightly to exposed the growth tip above the gravel. Another thing you can do is to insert root tabs to the gravel bed. It may help the plant to grow/adapt better as Echinodorus are hungry root feeders.
Cheers!
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