Validators can only pay out to 512 nominators per era. If more than 512 nominators nominate the same validator, it is "oversubscribed." Only the top 512 staked nominators (ranked by the amount of stake) of an oversubscribed validator can be rewarded.

In other words, you will not receive staking rewards if your active validator is oversubscribed and your stake is in place 513 or lower. 

How to Check If a Validator is Oversubscribed and if You Can Receive Rewards

On the Staking Dashboard, any oversubscribed validator has a warning sign and a red number next to it:

This red number is the minimum bond required to receive rewards for nominating this validator. If you nominate an oversubscribed validator, but your stake is above this minimum, you will receive staking rewards for supporting the validator. If your stake is lower, you will not receive staking rewards for supporting this validator, but you can still be slashed if the validator misbehaves.

You may want to change your nominations if one of your validators becomes oversubscribed. This article explains how you can do it.

To understand this topic in more detail, please check out our Wiki.

If you are more of a visual learner, check this video guide. You can skip to 10:58 to the part about oversubscription: