Perhaps solving this mystery means looking at the only thing that is different about the planters that thrive versus those that do not - must be the location. There are micro climates that make a difference just a few feet away so that if the sad planter is exposed to cold nights because it is less protected or dries out sooner under the eaves or most likely gets a blast of afternoon sun drying the roots. You did say you water every day but that could be too much water or not enough depending on the location. In a cool spot daily watering can be rotting the roots so check the soil by inserting your fingertip before you water. If the top one inch is dry you can water. Hope these tips help you figure out what is going on - there is always the less obvious culprits such as a pet using the planter as a bathroom area or drift from an herbicide. Keep growing, Marianne Binetti