Knowing now what plants they are, what the symptoms are, and some of the tank stats, I can already troubleshoot some of the problem.
Hornwort: Did you plant it into the substrate or float it? Hornwort does best when it's allowed to float because it likes A LOT of light, and it also likes a HEAVY nutrient level, which you don't have. Ferts may or may not help the hornwort, it will depend on what's in the fertilizer and what form it is in. Hornwort absorbs it's nutrient level directly from the water column instead of a root system like most plants. I have the stuff growing out of my ears within a month after putting it into my tanks... but I also stick to tanks with a heavier nutrient level... The last bunch I started was 12 inches long when I put it into the tank. When I pruned it 3 months later, that single strand of hornwort was then 6 feet long, soft, and bright pink on the tips. Anytime I have ever tried to "plant" or anchor the hornwort down low it has died. The deeper the tank, the faster it died.
For the lily, I will have to look it up... I know those symptoms but am not coming up with the cause off the top of my head. I have it in a book, will reply again after I get time to look it up.
With the val: They like to be CROWDED, so plant them in a group together, and if possible, increase their number by as many as you can. Groups of 5 planted close together with more groups of 5 will tend to thrive. Vals also like a lot of light, so be sure they are not shaded by other plants. The extra ferts should help the vals, too... but keep in mind, val is known for its heavy shedding, and is a "messy" plant, even when its healthy. You'll want to be sure to skim any debris from the surface of the water on a regular basis, as this can also effect nutrient levels in the tank.
What crypt are you keeping? Can you post a pic? Most of the crypts will do better with lower light, so if they are shaded by taller plants, they usually do pretty well. If it's one of the "red" species of crypt, the tablet fertilizers with Iron in them are the best thing to use. With the tablets, simply break 1 tablet into a few pieces and insert a piece into the root structure of each crypt plant once/wk. Serra makes a good one called "Flourenette A". It is the iron content in the plants that makes them red, and a typical tank doesn't have enough iron in it to keep the plants well fed and bright in color.
I'm not sure about the other plant you described... is it possible for you to post a picture for us? If I can identify the plant I can get the information about it, but I need to at least be able to see it to identify it properly. The onion plant family, aka Crinum, is usually a pretty easy plant to grow. It likes a bit less light than most others, like the crypts, and they can produce some awesome flowers at the surface when healthy and growing well. There is some good and accurate information here for you about the onion plants: http://www.kokosgoldfish.com/OnionPlant.html
I hope this was of some help to you, I will post again once I find the book about the lilys and what causes those symptoms. I know what page its on, just gotta find the book, lol.