I want to encourage you with a personal observation. Prayer is like being in the sun: you don't know how much it has affected you until the experience is over!
Often times, I have felt little to nothing during vocal let alone silent prayer, but afterwards I either received a powerful witness in my spirit that something great or significant had just happened, or (when the answer finally came) God let me know that it was in the "nothing hour" when the breakthrough came.
Let me also encourage you with something else I have come to realize: occurences such as keeping the mind from wondering or pushing through the distraction of boredom ARE NOT AUTOMATIC PROOFS of immaturity, rejection, or lack of effectiveness.
Prayer is work; and as with any duty or project, there are moments that are less desirable, less speedy than others yet are, nevertheless, part of the whole and must be undertaken. To endure through these, even if it means exerting more and strained efforts, is necessary; and you need to know that such times are just as acceptable to God as the ones in which you find personally enjoyable.
So keep praying. Keep working through. Keep trusting. Because like drilling for an artesian well, every hit, every exertion brings you closer to the resource God has hidden and preserved for you in Christ Jesus.