Humidity levels in new basement are high
Did it often rain in the last days/weeks?
Do the neighbours (new buildings) have the same problem?
Are all doors/windows in the basement closed during daytime?

As stated before, a new building needs to get rid of all the water in the concrete and other material. This can take many months, even if concrete may reach its final strength after 1 month.
A basement should be vented only during night time. The statistically dryer air can get in, will be heated a little bit during daytime (closed doors/windows), and thus conveys water out of the building, when this air will be replaced with dryer air from outdoor the next night.
This is also valid when using dehumidifiers. Windows/doors closed during daytime, open during night time.
All material which is to be dryed should not be covered with anything. The water must find a way to diffuse out of the material.
Incorrect drying of buildings is the most often mistake that causes huge damages to buildings - in the long run.