Hello @Creatorboxx,

Thanks for sharing your question with the Klaviyo Community!

From my experience this can often times occur if you have recently made changes to the flow and the time delays used within a flow. One thing to keep in mind is that changes made within a flow are not retroactive. This means that despite making updates to a flow, if a contact was already scheduled for a specific email, they would receive the email at that scheduled time even if the time delay and/or content of the email in the flow has been updated. For example, This is further explained in the How Contacts Move Through a Flow Help Center article. 

For example, if you had a two day time delay followed with an email immediately after the flow trigger and you decided to change the time delay from two days to one day; contacts who have already triggered the flow and are waiting in the two day time delay will still receive the email in two days. This would not force the email to be sent out any quicker. 

Another potential cause is if your flow happens to be a list triggered flow with double opt-in enabled in your list. A list triggered flow, also commonly known as a Welcome Series flow is triggered when a contact is first added to a list such as through subscribing from a signup form. When double opt-in is enabled on the list, contacts would not be added to the list until the double opt-in confirmation email is accepted. This means that despite a contact subscribing through from a signup form, if they did not confirm their subscription via the double opt-in email, they would not be added to the list and thus not trigger the flow. Because contacts would not be added to the list until their subscription has been confirmed, this also extends to contacts who opted in at a later time. For example, if a contact subscribed but does not confirm their opt-in status until several hours later, this contact would not trigger the Welcome Series flow until those several hours later after they confirmed their subscription. 

I hope this helps!

David