Isaac, why not try okbooks home page ad? I know it's discouraging and frustrating to spend your hard earned cash without the anticipated results, but that should not make you surrender. Remember, no good thing comes very easily.
Also, check the price of your book. Do not price your book high. Always keep it within the range of N100 to N500. Check your book description and cover page design. They equally promote books.
Okbooks is cooperative. I could remember when I advertised my first book, I paid N10,500 for both push notification and home page ad; an advert that supposed to run for only one week, but was left for complete three weeks. Though, the book recorded only 25 downloads within the peroid, but i was very happy because of their kind cooperation to me. I believe they will also cooperate with you.
You have taken the bold step to greatness. So, do not give up halfway. Keep searching for your audience through ad.
D. C. Chigbo