problem grouping/exporting shapes

I have a simple model built from 9 geometric shapes (all cylinders of different sizes). If I try to "Group" the shapes, one of them disappears. Similarly, if I try to download the model, the STL file omits one of the cylinders. It is always the same cylinder that "disappears." The preview image on my project page doesn't show the problematic cylinder, but if I open the model to "Tinker this," the cylinder is plainly there. Does anyone have a suggestions to fix this? 


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