Answer: This is a relatively pest and trouble free plant in general, and the symptoms you described would be consistent with over or underwatering. Since other plants are doing well, I would suspect a root problem. It is possible that for some reason the plant has failed to root out into the native soil and so the roots are still growing within the original potting mix.
You might try lifting it to see. If this is the case, untangle the roots and replant it.
Make sure it is planted at the correct depth, neither too high nor too low; to minimize frost heaving the crown should be up to an inch below the surface. If the shade is provided by trees, check and make sure there isn't a tree root invading the planting hole or anything else unusual happening below ground. Also be sure the soil drains well, the plant will deteriorate in overly moist or in heavy clay based soil.
Since we are in mid summer and the weather may be hot and dry, pour water into the bottom of the hole and then also water it as needed to keep the soil evenly moist but not sopping wet until it becomes established.
Another possibility, if the problem has been increasing as the temperatures have risen, is that the plant is in too much sun and consequently heat. You might try shading it slightly in late morning to see if that helps.
If none of the above seem to be the cause of the problem, you may want to take a leaf and stem sample as well as possibly some photos of the plant to your county extension to see if they can identify the problem more specifically for you.
Good luck with your Mint Frost -- they are gorgeous.
Q&A Library Searching Tips