Say that a proposition p is shifty provided that there are worlds w1 and w2 at which p holds, and a proposition q such that q grounds p at w1 but q does not ground p at w2.
A proposition p is non-shifty provided that either it cannot have a ground or there is some proposition q such that p entails that q grounds p.
The proposition that Obama or McCain is president is shifty: in some worlds it is grounded in Obama being president and in others it is instead grounded in McCain being president.
Propositions of the form of the proposition that N exists are non-shifty, if "N" directly rigidly refers to a substance.
Propositions expressed by typical simple subject-predicate sentences are shifty if trope theory is true. For instance, that I am sitting is grounded by the proposition that I have s1, where s1 is the trope of sitting that I actually have, but could have been equally well grounded by the proposition that I have s2, where s2 is some other trope of sitting. On the other hand, if Platonism is true, then sentences whose subject term directly refers and whose predicate expresses a property are going to be non-shifty, as in every world where they are true, they are grounded by some proposition of the form of the proposition that N exemplifies P.