I think that this is a glitch in the program. I've been having the same problem and I've heard of others also having this problem. It could be the video card. I had to change some settings to get mine to work right, resulting in not being able to utilize the cards full power ;o(. I think the programmers of poser need to look at the compatibility of the program with some of the newer video cards. What kind of video card have you got in your system? I'll have a scene just not want to render and get stuck as you have described, but when I ctrl/alt/delete out and open up that same scene again it'll, most of the time, render just fine. Is this similar to what you've experienced?