Why might the effect of Istikhara prayer sometimes be delayed, to the extent that Istikhara for marriage might end in divorce?

The outcome might be contrary to what the one performing Istikhara wishes and desires, but this does not mean it is not good, as Allah Almighty says: "And it may be that you dislike a thing which is good for you," and His saying: "And it may be that you dislike a thing and Allah makes therein much good." There might also be an impediment to the answering of the supplication, such as heedlessness of the heart or doubt. The answering of supplication does not necessarily mean granting the exact thing supplicated for; the supplication might be expedited, or stored for the Hereafter, or an equivalent evil might be averted by it, as stated in the Hadith: "No Muslim supplicates with a supplication in which there is no sin or breaking of kinship ties, except that Allah grants him one of three things: either his supplication is expedited for him, or He stores it for him in the Hereafter, or He averts from him an evil equivalent to it." The supplicant should not despair if the answer is delayed, for in the Hadith: "The supplication of anyone of you is answered as long as he does not impatiently say: 'I supplicated but I was not answered.'"