Clone stamp very erratic

[stephanie.vt]

I'm using Photo Shop elements 2.0. I've had great success with the clone stamp tool in the past so I thought I had the basics (select clone stamp tool in tool bar, alt click to sample area then drag over area), but suddenly it's not working at all. I drag over an area and it remains completely unaffected. I have selected "use all layers" and my opacity is 100%, so that's not the problem. One funny thing: when I un-check aligned I have more success but that's erratic as well. When I check aligned (which is really the mode I need to work in) nothing happens. I've tried closing the program and restarting. Advice appreciated