I have a problem occurring on a number of spreadsheets that I use regularly.
For some reason, the formula results are not updating correctly and are
showing the result as 0:00. All columns are formatted as [h]:mm.

Formula as shown =IF(C12<"",+C11-C12,"") appears in D12. This is the same
formula as rows above but rows above show correct result whereas D12 and
subsequent rows show only zero.

If I do an F2 and F9, the correct result shows in the Formula Editing bar so
formula is working correctly. I have tried copying both formula and cell
formatting from previous rows which do display their result correctly but
still doesn't fix the problem.

Options are set to Automatic Recalculation, doing extra recalcs has no
effect. One affected spreadsheet seems to have fixed itself when I closed
and reopened it.

Any assistance would be much appreciated