Depyrogenation Tunnels Convyor belt speed


I am hoping that someone can help me. Our company has a B&S intergrated vial filling machine. While doing some validation runs on the machine I found that the convyor speed increases although it’s set to stop at a certain speed.
This is a major concern since the ECV (Endotoxin Challenging Vials) proved a reduction of less than the required 3 log.

Does any one know why it would automatically speed up and do you have any suggestions.

I appreciate any help.