ok i finally got this problem solved, so here's the solution in case someone else needs it:
having a 'cmov' (compressed movie) atom means the movie resource has been compressed (ok that's really obvious!)
now this 'cmov' atom contains 2 other atoms:
1-'dcom' (data compression) atom
2-'cmvd' (compressed movie data) atom
dcom only contains 4 bytes for the size, 4 bytes for the atom type, and 4 bytes for the compression algorithm
(all files i came across had 'zlib' as the compression algo)
cmvd contains 4 bytes for the size, 4 bytes for the atom type, 4 bytes for the size of the uncompressed content, and the compressed content itself
so java standard library already contains a decompressor for zlib and voila, the uncompressed data contains the missing atoms