It's hard to say without seeing your document. Three possibilities immediately come to mind.
Clipping mask
If the objects are pasted at a point in the layers where they are effected by a mask, they'll only appear if they fall within the visible bounds. This is what a mask looks like in the layers list.
Everything within the top group will be cropped by the mask layer. If you ungroup the layers, the mask will be disabled.
Artboard bounds
Another type of masking that can bite you if you're not familiar with Sketch is grouped objects that fall out of the artboard's bounds. The artboard becomes a mask, just as if the object were partially over-hanging the bounds.
If you ungroup an object that lies entirely on the "pasteboard" (or if it was never in a group in the first place), Sketch will move it out of the artboard group so that it is not masked.
Sketch is glitchy
There's always the possibility that copying in an antique format like EPS may just be causing problems. Sketch has definitely taken the just ship it approach to development. They have a pretty focused use case (interface design) and they often break other workflows in the process of innovating.
