I cannot select cells from external worksheet while calculating

I am trying to create a simple calculation (as a test) within a worksheet that draws from an external worksheet. While doing this, LibreOffice is not allowing me to select the cells from the external worksheet. I have tried various methods to figure this out, but am unable. Any suggestions?

Which operating system do you use? Which file format does the external table and the table have in LO? How long does the calculation take? Is your computer busy with the calculation? Describe exactly what you are doing.

I found a work-around by having both worksheets not only opened but both minimized and showing on the same screen (o showing on two desktop screens). For some reasons the formulas allowed me to select cells from an external worksheet, unlike before, when they were both just open and not actually showing on the desktop.

Hope this helps (for those who may have been experiencing this same problem).

David