Your order could be declined for multiple reasons:
As an Individual, you may have:

• Insufficient funds to place an order. (If the price spikes before market opens and you do not have enough funds in your account to cover the share order, the order may be cancelled.)
• Caused a trade violation: Good Faith, Wash Trades, Spoofing or Layering. 

The Exchange may decline it because of:

• Instrument specific or market-wide circuit breaker
• Illiquidity of an instrument
• Volume issues with the instrument
• A reverse stock split
• Executing brokerage unable to execute the order
• IPO pricing is not available

Or;

• Fractional trading may not be allowed for that particular stock