Pv5 Earned Value field is blank

Can anyone help? I’ve a series of level of effort activities that are cost loaded in exactly the same way and are all baselined. All have budgeted costs, actual costs, cost percent completes... in fact there are no differences between them apart from name, id, dates. And in some of them the earned value column is showing zero - does anyone know why? I’ve looked all over the place to find a difference that might explain why one activity has EV calculated against it, but others haven’t - someone please point out that really obvious point that I’ve missed!!!

Thanks

Emma