This usually happens if the router has the PBC (Push Button Connect) activated. The wps pin is the correct but then you can't get the password. So you'll need to push physically the button on router.
Sometimes this behavior can be because a bad signal. You can try closer to the AP.
Other times it depends of the compatibility of your chipsed and the software. For example, if you have a Ralink chipset on your wireless card, Reaver doesn't work very well. If this is your case you can use the alternative to reaver (bully).
Another idea... Did you tried using -N (--no-nacks) parameter?
I found this, maybe it can help you: https://gitlab.com/billhibadb/reaver-wps/issues/203