The most common reason for this is the amount of memory allowed for PHP on your server. Please check and make sure it’s not 32M or less. You need at least 64M, but if you have other plugins 128M or even 256M is recommended.
Another possible problem is broken curl/sslcurl, please check CurlSSL from Help/Support tab.
Another possible reason is incorrectly setup WP Cron.
