You have a bottleneck in your visual attention.

When two distinct targets appear in rapid succession, you frequently miss the second one.

This is known as the attentional blink, where processing the first target uses up the available cognitive resources. In experiments with rapid serial visual presentation, participants miss a second target if it follows the first within 200 to 500 milliseconds. It shows that attention is not a continuous stream, but a series of discrete snapshots.