I;m running CRM 2013 SP1 on premises.
Today I noticed an interesting behavior:
Activity tab embedded on account for works just fine for some accounts and is always empty for a few others:

In case you cannot see the images above, here is a brief description:
On some accounts I go to the activity tab, click on add phone call and it just works. I can see the new phone call on the list right away. I can leave the page and come back and the list will be there.
For some other accounts I do the same but if I refresh the page, the list is gone. The activity tab is empty.
Same happens even if I follow the account.
I can always find the activities under the "Activities menu" but not on the activity tab on embedded on the account form.
Any help is really appreciated.
Thanks in advance.