Twitter-button not counting correct for some pages, while working on others


we have a strange behaviour with the twitter button: It’s not counting correctly for some sites, but works for other sites.

Example: – works – doesn’t work

As you can see, we use a two click solution, which only gets the numbers after the user first clicks on the button, but the code is the same on both sites, so this shouldn’t have any influence.
The number on should be definately higher, since alone from this account (@mexikokatalog) there are several tweets linking to the site.

All pages are hosted on the same server.
The above is just one case, we have quite some pages more with the same behaviour. ( and so on).