Statistic Field in a Report Returns Inconsistent Values

[ScottRobinsonPCMC]

I am trying to create a report and I have a statistic field ({SUM:ACRES}) that infrequently returns simply "{SUM:ACRES}" rather then an actual sum.  Also, that piece of dynamic text is supposed to return the "ac" suffix and it occasionally doesn't.  Can anyone point me where to look to try and fix this? I have attached a PDF containing screen shots of my field for reference.

Thanks.

[ScottRobinsonPCMC]

I think I fixed it.  the statistic field was in the group header.  When I put it in the group footer it worked.  Thoughts on that solution?