Why does available margin change during the trading day?

Margin availability isn’t static once you open a position; it’s recalculated continuously based on live market movement, since the value of any pledged shares or the risk profile of open F&O positions can shift throughout the session. If you’ve pledged shares as collateral, a fall in that stock’s price reduces the margin value you get from it (since margin is based on current market value minus a haircut, not a fixed number set at the time of pledging). For open F&O positions, SPAN margin requirements adjust in near real-time based on the underlying’s volatility, meaning a sudden volatile move can increase the required margin on an existing position even without you adding to it. Mark-to-market losses on open positions also reduce available margin as they accrue through the day, since unrealised losses get deducted from usable funds in real time rather than only at day’s end. You check available margin periodically through the day rather than assuming the figure you saw at market open still holds hours later, especially during a volatile session.