I tried to buy Sheng Siong stock but still pending. What is the reason?

You are likely using limit order. There might be a chance the limit order is cancelled if it is not fullfilled after a certain time period.

You tried to buy using market order or limit order?

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For limit order, it is possible that you don't get the stocks because you are way behind in the order queue and all the sell orders at the limit price had been fulfilled.