Amazon’s unionization woes are increasing, with workers at the company’s largest air hub pushing to organize.

Amazon has aggressively battled union organization efforts for years, even going so far as to deploy Pinkerton detectives to deter attempts. Despite its stance, support for unionization has been growing, and the company’s largest air hub outside of Cincinnati Northern Kentucky international airport is the latest site to experience significant union pressure.

According to The Guardian, workers are displeased with annual pay raises, with at least 400 of them signing a petition to have a peak season premium hourly rate enacted. Amazon normally pays its warehouse workers more during the holiday season, when sales reach their yearly peak but has yet to implement it at the NKY site.

Read more: Amazon Once Again Going Full-Press Against Unionization Efforts

“We have to operate a lot of heavy machinery, freight loaders, cargo tractors and things like that, and people aren’t paid any extra to do that work,” said Griffin Ritze, an air associate and ramp agent, and one of the organizing members onsite. “They just cross-train you in as many roles as possible and you’re constantly shuffled around.”

Workers have also complained that Amazon is not clearly communicating with them, including over things as serious as being written up.

“We do not have any clue that we are written up and never notified about it until we go to apply for a better position, that’s when we’ll find out,” said Steven Kelley, a learning ambassador at the KY site.

The employees ultimately make the point that Amazon depends on its warehouse and shipping workers as the lifeblood of the company and should therefore take better care of them.

“We’re the lifeblood of the company, not corporate, not upper management. We’re actually the ones who are sorting the freight, and loading the freight,” said Jordan Martin, a ramp associate at the air hub. “It’s the lifeblood of the company, the workers, who are actually organizing this effort and why we’re pushing for the better benefits that we’re trying to fight for.”