I'm new To FCP so this might be user error.

I captured some HDV from my FX1 and edited a small sample clip together, I'm pleasantly surprised at the performance of editing HDV natively [somthing I said I would never do].

Anyway I had some dissolves between clips and wanted to change the duration of the transition. When I made the change it didn't seem to have any effect. [there was no red render bar] So I deleted the dissolve from the time line but it still played it back as though the dissolve was still there.

Is there a cache I need to flush? Would it help to force it to render, if so how do I do that?

To force a new render: select the clip(s), and toggle visiblity off and on again.

"Ctrl-B" twice I think, from memory, or use the menu command twice. (I'm not at a Mac at the moment.)