What is wrong with my Drosophila culture?

After 3 to 4 days of egg laying on the medium exclusively prepared for the flies, many larvae develop well however, few of them grow up to the final stage.. I tried to propagate at room temperature but to no avail.. I resorted to the incubation at 29 and 27 degrees, still no signs of growing...Many of the young larvae died.. This is really frustrating for the spiderlings~~