Hi and welcome to ATP. Hope we can help you out.
It would help to know where you are, and what your climate is like right now - have you had a cool, rainy summer so far, or hot and dry? Has anything changed drastically in the past week or two?
Those look like fairly small plants, but you say "I usually don't have problems with my lavender plants, they are usually thriving". So are these new plants this year, or did you cut back old plants from last year? Or do you plant new ones each year? (that's what I have to do here in Florida because they just won't live through our humid summers)
Did you use a different potting mix than you have used before? Some of the "Moisture Control" mixes have that water holding gel stuff in them and can stay too wet for a dry climate plant like lavender.
