Last Visits and Return Visits wrong numbers



I´ve been having a problem with Last Visitors and Return Visitors since I upgraded the piwik in March version 1.2.1 to 1.3 and until now 1.5.1.
In February I had in average 20000 return visitors and now I have 500 return visitors.
Has anyone had this problem?

I´ve just attached the images to check the problem.

Raphael Milani