We have a Kohler thermostic value with two volume controls, one for the shower head, the other for the handheld. I like to use both at once and strangely when I do the temperature is significantly different between the two. (This was installed about 9 months ago. This has been true since the beginning but it seems to be even more true now.) My contractor didn't seem to have any idea why we were having this problem or what to do about it. Any thoughts? I like to use both at once and it's annoying that my shower burns while my handheld is tepid. Thanks,