One and a half gallon pots seem awfully small. If plants are in to small of pots they can get rootbound and simply can't get the water they need. I had a plants get so rootbound I had had to water it three times a day to stop it from drying out. When you water them does the water immediately run through the pot and out the bottom? If so they may not be able to absorb the water and nutrients because their is no place for the water in the pot.