This problem has been persistent for a while, just happens every so often at random times, nothing changing. I also do not know how to reproduce the problem, but I\'ll give a de
I found this solution, work fine for me:
In both spreadsheets insert an =now() equation in a random cell, say Z1 In both spreadsheets insert an =importrange() function that references the now function of the other spreadsheet. Go into your spreadsheet settings and choose to recalculate on every minute. I tried a lot of other suggestions including using the =now() function, the now URL trick in this thread, or Apps Script to insert random text on a set interval, but nothing would force importrange to update except a manual edit of the source sheet.
https://webapps.stackexchange.com/questions/60324/how-can-i-get-google-sheets-to-auto-update-a-reference-to-another-sheet