The most likely thing I can think of is that there is a little bit of dirt/sand/crud on the sealing surface of the shutoff. This can easily happen on new installations if there is something in the supply line/shutoff etc. Usually, if you take the shutoff apart, it can be cleaned off/flushed out, and then it can seal again. Not sure how that one comes apart. One of the pros will have some direction. You can probably find a parts breakdown on the web. Many of the Toto's use a Korky refill valve. Not sure about the one you have.