Pinholes can be a bit low contrast due to diffraction from the pinhole itself and flare from light bouncing around inside the camera.

As you are using Multigrade paper you could try slapping a multigrade filter over the lens to boost the contrast. In the darkroom you would use the MG filter (or magenta if you have a colour enlarger head) to print at different grades. I've never tried using a MG filter over the lens but it might just work! You can get them quite cheaply from ebay (or you could try some purple cellophane!!)