Derby won't shave

I just recently got a merkur 180 as my first DE, and used the merkur blade for a week and loved it. Had a little bit of tug, but nothing too bad at all. After the week I switched to a derby blade in my sample pack. First try with it, with a two day stubble, and it didn't shave. I stil saw my stubble on the area I went over. I then used my cartridge. The next day I tried the derby again and still nothing. The only time I get a shave is when I loosen the handle, and then I get a shave,but it's not real even and I nicked myself once. Does anyone have similar experience? Or does everyone have to loosen the handle some with some blades?