Couple of things I would check. Ultimately, the fan is not doing the work, but I don't know what the control system is for the fan, nor the process...so here are a few thoughts.
Have you checked for discharge piping/duct leaks? A large leak would exhaust the air and the fan may be doing less...
I have used Storm Technologies for this testing with good success. Good equipment, and competent staff. If you have everything ready for them when they arrive, this is a relatively straightforward test.
m
