Does ArcObjects ConstructUnion operation sometimes fail to merge touching parcels?

I have some code which identifies touching parcels and uses ConstructUnion to produce a single polygon from them. Just occasionally
the touching parcels remain unmerged even though they have been selected with esriSpatialRelTouches. Is this a known bug? Or,
the reason might be that I am using a geometryBag to store the parcels to be merged. The constructUnion documentation states
that it does not work on a geometrybag but I cannot find a single example on the web of anything else being used instead - can
anyone advise what I should be using? I am working with shapefiles.

Val