Callback is sent only once at the time of transfer bitcoins. There 0 confirmations. More callback is not sent. How to make a callback at 6 confirmations? After all, with 0 meaning no evidence of enrollment to make money as bitcoins may never come. Support is not responsible
I have had issues with Coinbase like this as well, they never admit it is their fault but I usually get callbacks around the 12th confirmation.
I would recommend using bitpay.com, they charge 1% (or as little as $30 a month + 0%) but they allow you to get a callback at 3 different levels, I believe something like 0, 3 or 6 confirms.
Coinbase payment processing is free, but you get what you pay for
A better alternative would be to make RPC calls to a bitcoin daemon you control.
You can get the paid status here: https://coinbase.com/api/doc/1.0/transactions/show.html
Check if the payment is Pending or complete and proceed. I do know that complete is the status after 6-7 confirmations.
Alternatively, you can use blockchain.info or Bitcoin RPC itself to get this information nice and easy. No other site can be checked for Coinbase confirmations as Coinbase uses an offchain network.