between 8 and 10 hours is pretty much the norm - any less and the plants suffer, any more and the fish do.
Im not an expert on all things plant but things to check/consider?
Water params - ammonia/nitrite can harm plants too
Oxygen/CO2 level - if you are running additional airstones, cos of med, or for areation, this will displace the CO2 from the water, which the plants need to breath!
Medication - anything been added to the water recently? Although most meds are plant safe some kinda cause plants to 'have a rest' and generally look dead until the water conditions are better
Food - are you specifically feeding your plants? While most will survive on the nitrates from the fish waste/left overs some hard to care for ones will need fertilisers, plant food
Lighting - Bog standard light tubes that come with tanks are sometimes only good for really easy/hardy plants. Look into upgrading the bulbs. Also, although they will still work, apparently most aquarium bulbs loose their frequencies after 6 months and are pretty much useless other than for visible light.
Location - is the plant planted properly, with a decent substrate and a good chance at spreading (up and sideways) and with space for root growth. If a plant cant gorw it will sometimes just give up and die.
Finally, what plants are we talking about?
I am sure if you can id them then looking into them on the net or asking in the planted section may bring you better results.
HTH
