Re-entering Cell Value

In some of my formulas, I need to re-enter the contents of the cell in order for a formula to compute.

For example, I am doing some math on a cell whose original contents are "10:21:00 AM" The formula returns a #NUM!. If I retype 10:21 in the cell and hit enter, the formula will now calculate properly. I have reformatted the entire column several times so that it is a custom hh:mm format. The original content also shows as time so I don't think that it is a formatting issue.

My worksheet has about 20,000 rows and this only happens for about 100 of the rows. It is pretty time consuming to manually go fix each one. Any ideas what is going on here?

Thanks