I've put together a spring assembly similar to the  "How to make an animation for a compression spring" tutorial:

http://grabcad.com/questions/how-to-make-an-animation-for-a-compression-spring-in-solidworks-2011

Which is sim to: http://www.youtube.com/watch?v=7PiuSpa-7QU

The initial setup works great. The spring updates to the top plate position during the entire animation.  Now I need to show this spring in an impact limiter assembly. I add a couple parts, everything seems ok. Then I an a sub-asm and then the rebuild during Calculate update stops. 

I delete the parts I added, still no rebuilt. 

I close the asm and reopen, no rebuild.

I close SW and reopen, rebuild works.

I ten replace the upper and lower plate with an actual part being used, replace the constraints, then the rebuild quits working......again. I replace it with the original part, no rebuild. Close and restart SW, rebuild works again (except still the old part).

At any time, I can force rebuild and the asm will update. But the auto-rebuild in the animation just stops working at seemingly random times. So how Mattias R got his asm to move so fluid in his youtube video seems impossible to me.

So I feel like I'm missing something obvious. Any suggestions?

SolidWorks 2012

Windows 7