Dental Cleaning Co-Pay

I have delta dental insurance basic plan through employer. I went for a dental cleaning yesterday and I was charged a co-pay. Do semi- annual cleanings now require a co-pay? When I read online, it says the preventative care should be 100% covered.

Update: I called insurance and provider. The dental cleaning should be covered. The provider said they did a comprehensive exam, which has a co-pay if you change dentist within 5 years.