If the plant was grown out of water then it can die when put under water.
If the plant was in a cold water pond/ tank and put into a warm water tank, it can die.
If the plant was in soft acid water and got put into hard alkaline water (or vice versa), it could die.
If they were getting 16 hours of light each day before you got them and now only get 6-8 hours a day, it can affect them.
Basically any sudden and massive change in temperature, water chemistry or lighting can affect them.
Perhaps try getting some from a different supplier.
See if the plants can stand up by themself when out of water. If they can, this indicates they were grown hydroponically and had wet roots but dry leaves. If this is the case, maybe try getting some that have been growing underwater. Plants grown underwater cannot support their own weight when lifted out of water and will collapse and hang down.
Ask the supplier what their pH, GH
, temperature and lighting times are. They might have them in alkaline water and the plants are suffering from pH shock due to your pH being low.