losing data in cell

I am using average calculation in a cell with varoius other cells linked for the numbers from which to calculate average. I do this in several cells. In certain cells the values drop out periodically leaving =(). While other cells continue properly.
I like Libreoffice very much and have contributed many times, but there is too much data lost for me to continue. I do not have this problem with other software. Thank you.

Please what’s your LIbreOffice version and Operating system?