Actual cost issue (n/w costing)

Dear experts,

please help me.

I'm using network costing in my project. I'm assigning work centers to activities (I'm using work center only to capture the actual cost). All workcenters have activity type assigned & a rate for each activity type is also defined. I'm using activity elements, where ever more detailed cost capture is required for a particular activity.

Say for example I have an Activity "XYZ" & I have assigned activity elements "abc1" , "abc2", "abc3" to the activity "XYZ". I'm confirming activity "XYZ" & activity elements "abc1" , "abc2", "abc3" by entering actual hours.

I click on activity/ activity element & press CTRLF10 (to calculate plan/actual cost) & then CTRLF11 (to display plan/ actual cost).

I can see all the planned values perfectly. I'm facing problem when it comes to actuals. I can see correct actual values for some of the activity elements/ activities. For some activities/ activity element system shows actual cost as zero. Why is it behaving like this? Any clues?

Thanks