Visio 2010 seems to be quite buggy with respect to operations. Worse, the behavior is inconsistent.
Earlier, I drew two concentric circles and two perpendicular diameter lines (which extended beyond the outer circle). I selected all items and clicked trim. This (properly) resulted in 8 arcs and 12 line segments. I just tried the exact same thing, and this time, the only result was the bisection of one of the diametrical lines.
Similarly, I found a suggestion for filling joined shapes which do not fill properly. Basically, you 'cover' the joined shape with a filled 'rectangle', select the whole mess and and then click fragment. This worked perfectly - once. It no longer works.
I'll try shutting down Visio and relaunching. But, this should not be necessary. Is anyone else experiencing similar problems? If so, have you found a solution? Is there some way to provide this feedback to MS?