Humm. Did you sent them the right .obj file after you exported?
Also, disappearing items can happen if you are putting these items in the normal object lists. For example, if you saved the Eruption under 'Miscellaneous' and then sent that .obj file to them, but their base Miscellaneous group does not have it, they wont see it on the maps.
To fix this, always put your new objects in your own groups. As in, put 'Eruptions' under a new object list, like MilksMiscellaneous.
<Acceptions include when someone makes an object file <like DreksObjects> and asks that you not sort them in your own tabs. But also, this means you don't want to put any other objects in this category so that anyone else getting a Dreksobjects.obj wont have any issues.>
That was long winded....but the basic rule is, new meshes should go under new object groups unless otherwise noted by the creator!