Your website must allow the HEAD HTTP request. Some security plugins (like BulletProof Security, for example) disable HEAD requests, which prevents the Twitter counter from incrementing correctly.

At the Linux shell prompt, you can use the following command to verify if your website allows HEAD requests. Replace "" used in this example by the URL to your website. If the command returns '200 OK', then your website allows HEAD requests. Any other return code means the request was blocked or redirected.

$ curl -I -A "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:23.0) Gecko/20100101 Firefox/23.0" 2>&1 | egrep '^(HTTP|Location)'
HTTP/1.1 200 OK

The second example shows a request that was blocked.

$ curl -I 2>&1 | egrep '^(HTTP|Location)'
HTTP/1.1 403 Forbidden