It actual looks like the client is sending data fast enough
waiting for empty buffer = data arriving faster than the storage device can write.
waiting for full buffer = data arriving to slow. The storage device is waiting for the next block of data to write
If not done already set :
SIZE_DATA_BUFFERS = 262144 (default value for some time time in NBU).
NUMBER_DATA_BUFFERS = 256 or 512
Tuning buffers is the number one tweak when dealing with tape.
The setting must be set on all media servers.
The buffer setting is written in the activity job log for each backup job.
There may be restrictions on the number of buffers if using FTL option i Netbackup.