bjb: This is a bug. Even after I upgraded to M020 I had the same problem. You have to push Ctr+F9 frequently.
And yes, there are calculations that refuse to update no matter how many times you push Ctrl+F9 and ctrl+R (Refresh). For these issues you need to delete the entire equation and re-input or, if it is a length bit of typing, you can shut MathCad down and re-open the spread sheet.
I was hoping this bug would be fixed in M030 but I cannot get the M030 upgrade. My one year maintenance ran out and despite all appeals PTC, their customer service insists I have to pay for bug fixes. We, the unwashed users, are not entitled to the bug-free software we paid for ... even though the coding errors are not our fault. This is a brave new, and unique, marketing strategy.
It is also a policy that will kill MathCad. I just purchased Maple 13 and I am in the process of converting my most frequently used work sheets from MathCad. Good bye to buggy software and cash cow, milk the customer, self enriching and arrogant support.