John,

You might check the soil. I've had a problem with soil drying out recently in the dryish air we've had (Never thought I'd be grateful for humidity)

ANyway, poke around in the soil with a chopstick. See if it's moist all the way through.

If that's not the problem, you might watch for the other problem--too much water. Both things can be a problem in hotter weather and symptoms are very similar.