There are a couple of things to check if you're having trouble changing your Lead's Funnel:
Have all the messages in the Funnel been sent? If the Lead has already received all messages in the Funnel or the Funnel time frame is shorter than the age of the lead, you will not be able to change the Lead's Funnel.
What to do: Assign the Lead to a different Funnel or one that matches the age of the Lead.
Are the messages in the Funnel disabled? If the messages in the Funnel are disabled, you won't be able to change your Lead's Funnel.
What to do: In the case that your Funnel is disabled, you will need to make sure that at least one or all of the Emails in the Funnel is enabled. Click here to learn how to enable the messages in a Funnel.
Is the Lead's email address valid? If the email bounces or the Lead unsubscribes from your Funnel, you may not be able to change the Funnel. Learn more about turning off a lead's Funnel:
What to do: We recommend you turn the Funnel off for that Lead. We have instructions on turning your Funnel off in this article.