I recently made my own monero pool. While setting up the minimum payment threshold I thought: What happens when a miner doesn't reach the required XMR Balance? It might seem like a simple question but the miner did submit some shares to the pool. When the pool finds a new block it needs to reward all miners based on how many shares each one submitted. When paying out does it take into account the shares of miners that didn't reach the required balance? My guess would be that it takes into account all the shares, pays out the miners that reached the minimum payment threshold and keeps the excess XMR until other miners reach the required balance and then pays them out as well.
When a block is found, the block reward is divided among the miners, pro rata with the difficulty weighted shares they submitted for this block. This is done by the pool software's own accounting, not on chain.
On chain transactions only happen when a miner's pending balance reaches the payment threshold. At that point, some amount (based on the denomination set in the pool's configuration) is sent to the miner, and any remnant is kept in the miner's pending balance.
This pending balance is not per block: it is permanent, and a miner mines to it, block after block. Therefore, if a miner's share of a given block is below the payout threshold, the balance (in the pool's accounting) increases by that small amount, and counts for future blocks. This means that small miners will get paid something every N blocks found, while large miners may be paid something every block found.
With many pools, if the PoW earnings don't meet their minimum for payout, you can never collect. There may be thousands of miners in any given pool. Especially with alt-coins (I am one of them). I've often wondered what happens to the collective unpaid earnings, which, at the high coin values these days, could total very substantial amounts. I don't know how a pool owner could do anything with these earned coins. This is an even bigger problem with cloud mining. It seems there should be a means for miners to "cash out" their uncollected earnings. If not individually, then as a "pool". Then the miners could collect most or all of their earnings, perhaps subtracting a fee from the pool operators, or an arbiter who oversees this. This may be a potential legal challenge facing virtually every pool operating today.