There could be a number of reasons your plants are dying. Not enough light for the type of plant, planted incorrectly, being eaten by fish for a start. If you have just pulled up the remains without giving the gravel a good vacuum, there may be bad bacteria built up where the plant remains are. The plec would be hit first as it is a bottom dweller.
Give the substrate an extremely good and deep siphon.
Have you tested for ammonia, nitrite and nitrate?