Can't get the number of Activities in a report

Hi, I need to make a report with 3 entities: Account, Custom Object 7 and Activities. So, I start choosing the Advanced Custom Objects Subject Area. Everything works fine when adding the Account name field and some fields from the custom object that I need to show, but when I try to add the # of Activities I get always 0 in all the records. It seems that the tool can't obtain that field data. Can anyone help me If I'm doing something wrong?

I'm very inexperienced doing reports and doesn't understand well the rules or best practices to make reports, but in many cases I have that problem: when trying to build a report with more than 2 or 3 different entities is impossible to get the data from On Demand.