It would probably be easiest and fastest to simply request addition testnet coin from a faucet. I'd suggest TP's TestNet Faucet which at the time of this writing is giving out 0.94 tbtc per request.

If this also doesn't work, it's possible that Coinbase's testnet node is down, in which case you'd have to contact their support team.

Please remember to return your tbtc to TP's Faucet once your done with it!