It occasionally happens that a cell looks blank (i.e. null), but it is a blank, not a null.  You'll know this is the case because writing if statements using 'is null' don't work.  There is a simple fix for this.  Instead of writing something like this:

if([PlayName] is null, "Null", "OK")

...write this...

if([PlayName]="", "Null", "OK")

The screen shot shown is using the formula with the quotes to indicate a blank.