Why are some transactions found on some chain explorers but not on others?

This is something that has puzzled me for a while and I cannot think of a rational reason for it. The transaction 3361d2484f0566ab13d32f2ab321319945f48eaea1ac2cc9f5a79b40528359c3 shows properly on blockchain.info and tradeblock.

However, I cannot find it either on blockr.io nor blocktrail.com.— I am also running a server to watch transactions and I cannot find this transaction either. Sometimes I can find transactions that do not appear on blockchain so it’s quite random.

All servers have many connections so I wonder what could be the issue? Slow network?

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