I just miss it for lack of attention. When I pray with some intentionality, I should immediately anticipate God's answer (whatever it might be). Most often, though, I pray and immediately forget about what I was praying about. I forget to then look for how God may be answering that prayer. I ignore and miss all the unusual things about my day. I see them as menial or distracting, and fail to connect them with the activity of an omnipresent, sovereign, omniscient God I had JUST spoken with.
When I pray I need to immediately begin watching and listening to what happens next. Be prepared to make adjustments to MY plan. The thought that God is not going to answer my prayer should never cross my mind.
God actively answers, I just rarely stick around to notice.