Why is my limit order not executed when the current price is as per my limit order?

Limit orders are executed on a first-come-first-served basis. This means if two limit orders are placed at 10 am and 11 am respectively, then the one placed at 10 am will be executed first. And depending on the market, the order placed at 11 am may get partially executed, or fully executed, or even not executed at all.