You're over watering them. The plants don't mind getting a little dry inbetween waterings. As a general rule, you can stick your index finder in the soil down an inch or 1.5 inches; if there is any moisture at all, dont water. If it's bone dry, go ahead and water them. The reason for this is that when you water, you want to water a large amount so that the water can reach not just the roots near the surface, but also the ones that have worked their way deeper into the soil.
If you water the plants more often but with less amount of water, the water will never get down to the bottom roots but instead more likely than not swamp the roots closer to the surface. If you do this too often, you can soak the roots and waterlog them which leads to root rot.
It's hard in the beginning trying to figure out when to water the plants, but try the finger rule. Also, you can put your plants on a weighing scale and water them based on how much the weight has dropped. Keep in mind that your plants are more likely to survive a dry spell than overwatering them, so be careful!
