Why does our porta potty smell like ammonia after a week on site?

We have a standard portable toilet on our residential addition site in Chicago. It's been there for three weeks with weekly service, but by the end of each week, it smells strongly of ammonia. There are 8 crew members using it daily for 8 hours, no food or alcohol on site. The unit sits on level gravel, and the vacuum truck has clear access every Monday. The rental company uses a formaldehyde-free deodorizer. Is this ammonia smell normal for a unit after a week of use, or could it be something else like the deodorizer formula or the service not being thorough enough?