For all of our pools we pay every hour to all miners that have a payment threshold larger than the pool minimum threshold. On some pools we set the default threshold for exchanges higher to relieve the load on the exchanges, but you can log in and adjust this.
As the pool finds blocks, they go through a maturity process. This is to make sure there are no problems on the network, and all the network nodes agree that the new block is submitted and confirmed.
Once a block has been paid, your total due will increase until it is larger than the pool minimum threshold, at which point the next time the payment system runs, your wallet will be included in the payment run.
Sometimes due to the way Cryptonote works, it is not possible to make payments to everyone in the queue. To explain why - when you make a payment from one wallet to another, more coins are taken out from the (pool) wallet than the actual transaction payment amount, and then the change is returned after a few blocks on the network.
Very simply put, think of it like buying some cookies for 50 cents and handing the cashier 5 dollars and having to wait half an hour for your change - you can only buy so many cookies before you have to wait for your money back, so you can buy more cookies.
At times, it can add a delay to payments. There is nothing wrong; you will get paid eventually. Delayed Cryptonote payments from the mining pool mostly become an issue when people mine directly to exchange addresses, and a group of miners reaches their threshold at the same time (when the pool gets lucky usually).