It is possible that the plants are growing too large for their own good and require a little pruning, as mentioned.
However, you did not mention what kind of plants you are trying to grow emersed. Some plants might be a little more finicky than others, so they may die back a bit.
When I was growing Cryptocoryne species submersed, I had the feeling that if I looked at them wrong, they would melt. :icon_roll