I hold a call warrant on HSI. I understand the call warrant price should drop as the HSI drops. However, why did the liquidity provider lower the bid price only but not the ask price. This deprives my chance of buying more call warrant at a lower price.

Liquidity providers can provide bid and ask quotes at the pricing level they deem fair by reference to the theoretical price of the call warrant provided that they comply with the maximum spread requirements. The bid and ask spread may change from the time to time according to the prevailing market conditions affecting the underlying asset such as hedging costs and liquidity, spread and volatility of the relevant hedging vehicles. Therefore it is possible that their quotes will not match your expected price levels due to the widening of the bid and ask spread.

For example, we assume that the theoretical price of the call warrant was initially HK$0.2 with a spread of HK$0.003 on each side, such that the bid and ask quotes were initially HK$0.197 and HK$0.203 respectively. The HSI then drops which leads to a corresponding drop in the theoretical price of the call warrant to HK$0.197. Notwithstanding the drop in the theoretical price of the call warrant, the spread may be widened to reflect the prevailing market conditions. For example, if the spread is then widened to HK$0.006 on each side. The new bid and ask quotes will become HK$0.191 and HK$0.203 respectively for the warrant with a theoretical price of HK$0.197. In such case, the ask quote does not change but the bid quote is lowered due to the widened spread.