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Are you sure Bitcoin transaction is done on luck based?
to some extent yes, is is based on your luck.
you may open bitcoinfees.21.co and see suggested fee is for example 300 satoshi/byte and pay that amount but then see 2 or 3 blocks being mined and none of them include your transaction. simply because there is a spam attack and there are a lot more 300 s/b and more in the mempool and on top of that these 2 or 3 blocks were mined by one of those miners who fill the blocks with their own crap.
like what BitFury does with their Op_Retrun transactions which sometimes even takes half the block size.
Those are just a guide lines what is happening in blockchain and while choosing fee you also should look at how many unconfirmed transactions in the blockchain. If you see a lot of unconfirmed transaction means you shouldn't follow what they suggested instead pay a slightly higher fee than what they suggested to get your transaction confirmation faster.
I do agree that there will be some delays but 24 to 48 hours is too much that shows the fee paid for those transactions was way below than the expected fee.