Hi David...
On rare occasions you'll get an oddball feature that just doesn't behave. Often I'll just delete and recreate that feature. In the event that this causes major headaches, there are a few things you can try.
My first bet would be to increase the accuracy of the model. Go to File>Properties and jack the accuracy from the default of .0012 to something like .0003. This will force a full regeneration of the model that can sometimes fix oddball problems like this.
In a pinch, a second technique would be to save the file to a new name. Open the new file and see if the same weird behavior continues. Sometimes this alone is enough to fix the part. If this works, rename the old part to "partname_obsolete" and rename the new part to the "partname". You can perform some voodoo with Windchill or Pro/INTRALINK to replace the file, too.
If the same problem exists in the NEW file, you're likely going to be stuck having to delete the bad feature and recreate it. Suppress the old thicken (and all children). Re-create the thicken (as a new feature). Unsuppress everything and delete the "bad" thicken. Suspend the children and reroute their references to the new "good" thicken.
On the off chance that this part is somehow being created from an assembly (or referenced from an assembly), check for things like circular references, or other failed/frozen geometry which could be contributing to the problem. These are highly unlikely causes but they're worth mentioning.
Good luck. Let us know how things turn out!
Thanks!
-Brian
