You can collect the diagnostic to check what's going on behind the scenes using following metalink note.
*Troubleshooting RMAN Performance or Hang Issues [ID 815857.1]*
Also, How many total number of CPU cores you have on you machine? Oracle will not spawn channels more than our total number of cores at one single point of time. So in your case it might be possible that your all cores are active backing up datafiles and just waiting for them to finish before going any further.
100 channels seems like overkill. Do you really want 100 separate processes concurrently doing I/O ? Where are the backups being written to ? (e.g. if tape, then you really need very many tape drives, if disk how are you spreading the write I/O of those 100 channels ?)