Well, first of all, it's a 24-hour
rolling limit. AIUI, this means that the buy window is open for however long it takes until the limit is hit again. Depending on demand, this could cause the starting/ending times of the limit to shift.
Several people
(myself included) have resorted to using automatic "buy" scripts to try and force an order through the minute the limit lifts. This has probably caused the window of opportunity for a buy order to be very short-lived. Sorry, you do what you gotta do!
FWIW, I bought just fine on a downturn
(order went through at ~$85 USD / BTC as it was on its way down to ~$50 USD / BTC), although it took ~600 attempts over the course of ~10 hours.
Maybe the limit lifts prematurely if there were recently enough sell orders to put BTC back in their coffers? That's totally unfounded speculation, though.