John,

There's a few possibilities:

1) Hit F9>Options. If "Make Open ended activities critical" box is checked, activities with no predecesor will be coloured red even if they have float

2) From the same Options window, if "Define critical activities as" is set to "longest path", then only those activies on the longest path through the project will be shaded red, so if another activitiy not on the loogest path has zero or even negative float because of a finish no later than constraint for example, it will not be coloured red.

3) If instead "Define critical activities as" is set to, say, float less than or equal to 0 hours, and your total float column is displaying float in days, you could have activities with, say, 2 hrs float that are not shaded red, but are rounded down to 0 days float in the column. You can check this by going to Edit > User prefs > Time units, and making the duration foramt the same unit as you have used in the "Define critical activities as" option

4) Someone may have played around with the bar settings, so the standard "Critical remianing" bar (which is the one shaded red) isn't displaying properly becuase it is set to the wrong order of prsecedence, or the filter has been changed or something.

Cheers,

G