Pool gets rewarded for producing blocks. If you want to receive rewards in every epoch, you need to find a pool that has a large enough stake. Pools with a low stake will not get a chance to create a block in every epoch, so they will not get a reward. It is not smart to delegate to a pool that is over-saturated. To prevent existence of big pools, there is a limit to the size of rewards a pool can earn per epoch. If the number of ADA coins in the pool exceeds what is considered the saturation point, the rewards for the pool operator and stakers begin to diminish. The Cardano network is designed to reward stackers approximately 4.2% of ADA coins per year from staked ADA coins. Large pools will be more consistent in block production, so rewards will be more consistent. Smaller pools may have more variance in the amount of blocks produced, so rewards will also have variance. It may be the case that rewards are higher than average in one epoch but lower in other epochs. If the pool is lucky, the rewards may be higher in more successive epochs. It is important to note that the performance of the pool must be observed over longer time and that the yield over longer time periods will be close to the average, i.e. 4.2%.
