If you are using distilled water, yet are using the same fertilizer you had been using with your "regular" water supply, it is likely that the pH is dangerously low.
Most water is pretty well loaded with minerals, so is buffered by them, and the types of chemicals used in the fertilizer - actually the balance between nitrates and ammoniacal-based minerals - makes little difference on the solution pH, so they simply go as cheaply as possible. Use a water supply without those dissolved minerals - distilled, RO, deionized, collected rainwater, etc. - and there is no buffering action, and the pH will change drastically depending upon the chemicals added. Pure, degassed water, for example, has a pH of 7, but even absorbing a minuscule amount of carbon dioxide from the air drops it to about 5 (it is a very weak acid, though).
You basically have three options:
- Go back to your tap water and use the fertilizer you have.
- Use that fertilizer with the distilled water, but adjust the pH upward to the mid-5's to mid-6's range.
- Switch to a fertilizer designed for use with pure water.
Some will tell you to mix some of your tap water with the RO in an attempt to reintroduce the buffering minerals, but I don't see how you can know how much to add.
If you have not been feeding them at all, you are most certainly starving the plants, and the pH issues can still arise through the interaction with residues in the medium.