Child Asset cost does not rollup

Hi, In R 12.1.3 one user has added several children to the parent asset. In the first transaction performed the system rolled up into the parent asset the child cost and reserve ( leaving the child asset with "0" value). In the second transaction the user performed the roll up did not happen. I have investigated if anything different between the 2 transactions and both have similar dynamic: child assets are not end dated, they all have been added to parent asset in the same month...Any suggestion on what else I could check? Thanks , Monica