That's just how bitcoin works. Sometimes it's because the transaction didn't include enough fees and the miner didn't pick it up. It could be because the transaction didn't propogate enough to reach the miner. It could be that it uses an unconfirmed input etc. That's why we add a generous 0.0005 BTC fee when we pay the winning transaction, to get the miners to pick up our transaction, plus the previous transaction, in case it didn't include enough fees.