I've paid the seller but I haven't received my Bitcoin yet. Is this normal?

The sellers usually release Bitcoin as soon as they see your payment, which can sometimes take an hour or two. If you've paid but are still waiting there is nothing to worry about, your trades are protected by the escrow and the seller cannot run away with your Bitcoin. 
If there is any issue with a trade and the seller won't release your Bitcoin, you can dispute the trade to get the CoinCola support team to resolve it. 
If you are buying or selling Bitcoin, you can dispute the trade 30 minutes after you have marked the payment completed. 
A dispute can no longer be started if the Bitcoin has been released from the escrow. 

When a trade you're involved in becomes disputed, you'll receive a notification. In most cases simply starting the dispute is enough to get the attention of your trading partner. A disputed trade is usually resolved within 12-48 hours.