I constructed my own TK from the WTF ANH kit, and I also found some of the areas to be a little tight. By chance, do you have return edges around the openings to the top and/or bottom of the bicep? If so, that may be contributing to the issue (if the problem is when putting your arm in and taking it out). As you can see in some of the reference photos that Glen posted, the armor used in the film lacked return edges in many areas, including the bicep.