As your list grows, the mix of readers changes. Some people join because they are excited, while others join out of curiosity and then lose interest later. Not everyone checks their inbox the same way, so open rates shift even if your content stays the same. When you send one message to a large group with different needs, it won't hit everyone the same way. Some may open everything, while others barely notice your updates. This makes the numbers look lower even though nothing serious is wrong. Breaking your list into smaller groups can make the message feel more relevant again. What do you think makes people stop opening emails after some time?