Need help! Visit breaks up in mutliple visits

Hello!
Recently I have the problem that visits break up into several visits and are then counted multiple times in the stats.

See the attachted example. Piwik recognizes the page views (which are within seconds/minutes) as coming from the same visitor but counts them as separate visits.

Does anyone know the reason why or what can be done about it? The site is running on wordpress and uses WP-Piwik to add the tracking code.

Thanks!