Actually bestowed grace would as a rule seem to be irresistible, unless it is a part of a series of soul-designing events, such that even if some grace in that series is resisted, nevertheless, the entire series accomplishes God's end which moreover could not be achieved with less unpleasantness or greater efficiency than by allowing that grace to be resisted.
Another possibility is that God may supply a grace that would be poorly received for the sake of some greatest good for the greatest number.
The only other reason to bestow a grace that is resisted is in order to have a better "legal" reason to condemn a person, which is nuts. God is a maker of gods not devils.
As the Catholic Encyclopedia relates, Jansen
did not shrink from reviling sufficient grace, understood in the Catholic sense, as a monstrous conception and a means of filling hell with reprobates, while later Jansenists discovered in it such a pernicious character as to infer the appropriateness of the prayer: ... "From sufficient grace, O Lord deliver us."
I agree with the Catholic opinion, even though they do distinguish sufficient and efficacious grace.
Further, to think that grace can fail to do what the omnipotent and omniscient God wants it to do seems absurd, as well: grace
may be considered, secondly, as it is from God the Mover, and thus it has a necessity -- not indeed of coercion, but of infallibility -- as regards what it is ordained to by God, since God's intention cannot fail...
Hence if God intends, while moving, that the one whose heart He moves should attain to grace, he will infallibly attain to it... (ST, II-I, 112, 3)
Natural virtue is a necessary condition for being eligible for grace, and it is also a quasi-sufficient condition -- for God is so good that He won't miss any opportunity for uplifting a person into deiformity. The Catholic Encyclopedia agrees:
God, out of mere liberality, does not withhold His grace from the one who accomplishes what he can with his natural moral strength, i.e. from the one who, by deliberate abstention from offenses, seeks to dispose God favorably towards him and thus prepares himself negatively for grace.
Some theologians... declared even this most mitigated and mildest interpretation to be Semipelagian. Most modern theological authorities, however,... see in it nothing else but the expression of the truth:
To the one who prepares himself negatively and places no obstacle to the ever-ready influence of grace, God in general is more inclined to offer his grace than to another who wallows in the mire of sin and thus neglects to accomplish what lies in his power.
In this manner the cause of the distribution of grace is located not in the dignity of nature, but, conformably to orthodoxy, in the universal will of God to save mankind.
It may be objected to this reasoning that "Those who are well do not need a physician, but the sick do. I did not come to call the righteous but sinners." (Mk 2:17) Very well, grace for the sake of the individual receiving it can be divided into common (which heals nature) and sanctifying (which uplifts nature), e.g.,
For one [love] is common, whereby He loves "all things that are"..., and thereby gives things their natural being.
But the second is a special love, whereby He draws the rational creature above the condition of its nature to a participation of the Divine good; and according to this love He is said to love anyone simply, since it is by this love that God simply wishes the eternal good, which is Himself, for the creature. (110, 1)
I expect that most graces bestowed by God are in fact not resisted, since in the event that God foresees that a grace would be rejected, He would refrain from giving it in the first place.