Resource limit problem in resource usage profile

hey, i'm using P6 Professional R8.1, when i check the resource usage profile i find drops in the limit line - limit=Zero - in some working days! my calender has only a weekly vacation on Friday so the limit is dropped to zero in Fridays and that's logical for sure, but i don't know why it's a zero in some working days! i tried to add new resources with new calender where no holidays at all and still the limit line dropped in some days! how can i solve this problem, specially cause i need the resource usage profile printed so they must be correct.

here you're a photo of an example, this zero-limited Monday is a normal 8h working day