[Koha] Rebuild-zebra stops working very often...

Dear all I have got some problems with rebuild-zebra which stops working very often, as a result our librarians can't find the newly entered items (the search indexes are not up to date). When I manually restart zebra, it solves the problem. But then it stops again running the next day, sooner or later. We just don't know why it stops working. The person responsible for the server said that << The logs suggested that the re-indexing was being run according to the schedule >> Do you have any idea why it would stop working? What should we check? Where could it come from? Many thanks for your help! Sonia.