How do you water your plants? Is it enough to reach the roots deep in the pots? I wonder if they are being given enough water. Before you water them, have a feel of the soil around the plant. Stick your finger into the soil to determine if the soil is dry or moist a few inches under the surface. you may need to water them more deeply and/or increase the amount of water you are giving them if the conditions warrant it. The other thing it could be is burning from fertilizer, particularly if they are not getting enough water. Have you fed them with anything?