I've bred at least 10-15 Pichus now, all with Volt Tackle and so far each one has come out as a female. I'm playing Emerald and I've tried with the Light Ball in both the male and females on separate occasions.
Pichu has 50% for each gender. There are no methods to downpass gender.
Maybe you just getting bad luck. For each pichu you breed you have 50/50 to be female or male.
Hope this helps. <.< (Also thanks to Immortal and Kawaii for helping me.)