i also have had problems uploading mesh at times, only advice i can give is
only use the sl linden lab viewer for uploading mesh, i've had problems with firestorm viewer not uploading correctly, i have not tried the latest releases of firestorm,
make sure you unwrap your mesh
and also for multi mesh uploads, make sure to rename each uvmap a different name, uvmap1, uvmap2, etc before uploading,
i've had error messages when i try to use the same complicated mesh as my physics shape, but if i use a simple physics shape with alot less vertices, it seems to upload with no problems, dont know why this happens