collection item's number (strength) = 0 but collection not empty

[Compte Messagerie]

hello,

i have dspace 6.3,  it functions normaly until last month, where some collections in just one sub-community, some and not all collections of the sub community show [0] items like in the picture on attachement . and when users click on to see the list of items it shows an empty page like in the second attachement.

In the data base, the items of the empty collections  exist ,  and i compared an old assetstore (backed up previously) with the current one and there are no additional folders in the old assetstore that don't exist in the current one.

when i search an item of one of these collections it doesn't  return any result

what must i search in logs?

what can i do to solve this issue?

 thanks