If find it really aggravating to fix a problem only to encounter it again -- after I've long forgotten how I fixed it. This happened to me when I was creating a new PHP wrapper class around the Flexigrid plugin for jQuery.
And the reason was ... the stylesheet had:
table { width: 100%; }
Remove this or add a class or inline style to your table to allow Flexigrid to do its thing.
Of course, I don't actually remember if this is the problem I encountered last time, but I'm posting this for the benefit of all -- including myself.