when i set isKinematic to false in a script it doesnt do anything it just floats there. i had this problem before in 2.6.1 but my reason for using this was not importent then but it is now.
Is useGravity set to true?
yes. by turning it false from the inspector it will fall. if i do it from a script it doesnt. though if something hits it (the something probably has to have a rigidbody) after i turn it to false it will fall (this worked in 2.6.1 have not tried in 3). my current solution is after making it false applying a tiny bit of force to the rigidbody.