> only for it to go over the heads

If it actually went over their heads, then the effort was wasted. I've heard the goal of preaching described thus: "Address the mind to move the heart to change the will." If you haven't addressed the minds of the people you're speaking to, your preaching was a failure.

NB if the people in the parish don't want to change their will, and so close up their minds, that's a different issue.

Indeed. As one priest in graduate school said to me (and with which I agree), one should generally keep homilies short, simple, clear, and to the point. In most cases, it isn’t the proper place for an extended theological meditation.

Of course, people ought to realize that the purpose of the mass is not the homily, but the sacrifice of the eucharist, which is the “source and summit of the Christian life”.