60 and 75% is ideal...

If the humidity tray does not raise it as much as you want, if you direct the fan at the tray it will spread a lot more moisture so one way or the other you will get it up if you want to.

Also when I first set up my case I was using a humidity tray, once my case was set up and filled with 60 orchids I had to start leaving the door open a little so it might be you have too few plants in there ;) Every time you add or take away plants things will change slighly. The more plants you have the more they will raise the humidity themselves.