closing geometry
E-mail me a file that has the problem that you speak of and I will correct it for you and tell you what I did and how I did it.
Basically, you must check every intersection for a break or an overlapping line, by zooming in as much as you need to in order to see it. Where lines connect with radii, you must correct by using the Tangent Relation tool,etc. If a line is too short in connecting to another line whether it be perpendicular, or continual, the Extend tool is very handy. Another thing you must check for is double lines, by that I mean a line that you may have put in over another line then forgot to delete the first one. It isn't a very difficult procedure. If after you do all of the above, then exit the sketch, and select it, and select the extrude tool, you will know immediately if it is kosher or not by the presence or non presence of the yellow preview lines.
Ben Alba