We're sorry to hear the Send to Kindle feature appears to not be working for you.
On occasion we get reports of this happening, and because it is a matter of using a third-party service (i.e. we have no control over how Kindle works, we can just send emails), there is little we can do ourselves, besides provide advice for other ways to get your Leanpub book onto Kindle.
Update: One reader who encountered this issue shared the following (lightly edited) information with us, when they had this issue:
Turned out that I had changed my Amazon password, but didn't update it on the Kindle. I didn't realize the Kindle didn't update, until I tried to send a document from Leanpub, which made me think initially the problem was with Leanpub.
If that's not the solution in your case, here's an article from our Help Center with some information about how to do this:
Please note that we always send books to free.kindle.com - there's a (very wordy!) explanation of it on the Kindle page https://leanpub.com/user_dashboard/kindle that looks like this: