What the Molinists defend: the act may fail to follow upon all prerequisites; Reformed and recent Thomists deny it
What they do defend, and strongly, is that the will is in itself so undetermined and indifferent that, given everything required beforehand for acting, whether on God's side, the object's, or the intellect's, the will can fail to act, and often actually does. Nothing, then, that moves and stirs the will to act beforehand is such that, once it is in place, the will's act cannot fail to follow; quite often it really does not follow.
So everything that precedes the will's act and is required for it can really be combined not only with a power not to act or to do something else but with the very absence of the act, and even with the contrary act.
As the foregoing shows clearly enough, both the Reformed and those more recent Thomists deny this. They hold that a power not to act, or to do the opposite, can indeed coexist in the will with what God's side or the intellect's side requires for acting, but that once those are in place the will always acts and nothing else can happen. On that supposition the will acts surely and infallibly, though it keeps in itself the power not to act.
Latin
XXXIV. Sed acriter defendunt voluntatem in se sic indeterminatam & indifferentem esse, ut positis omnibus ad agendum praerequisitis, sive ex parte Dei, sive ex parte objecti, sive ex parte intellectus, fieri possit & saepe etiam eveniat, ut voluntas non agat: adeoque nihil esse eorum quae voluntatem antecedenter ad agendum movent atque incitant, quo posito non possit voluntatis actio non sequi, & saepius etiam reapse non sequatur: & sic cum omnibus illis quae actionem voluntatis praecedunt, & ad eam praerequiruntur, revera conjungi & componi posse, non solum potestatem ad non agendum aut aliud agendum, sed ipsam actus carentiam, & actum quoque contrarium. Quod negant, ut satis liquet ex antedictis, non solum Reformati, verum etiam Thomistae illi recentiores. Quippe sentiunt cum iis quae vel ex parte Dei, vel ex parte intellectus ad agendum necessaria sunt, posse quidem in voluntate consistere potestatem non agendi, vel agendi oppositum; sed tamen iis positis voluntatem semper agere, nec fieri posse ut aliter eveniat: quoniam ex hac hypothesi voluntas agit certo & infallibiliter, licet in se retineat non agendi potentiam.