Mamor, very rarely is this caused by the ejector. There are 3 primary reasons for this type of jam. 1)Load too light for recoil spring weight. 2)Extractor tension too light. 3)Insufficient grip on gun. Since you also mention that sometimes they don't make it to the stovepipe position (caught between the slide and breech)it almost has to be your extractor. If you will go to
http://www.m1911.org/technic.htm and you'll find an article written by Bill Wilson on adjusting your extractor. If after reading it you don't feel comfortable doing it yourself, either take it to your local smith or you can e-mail me. I'd be happy to do it for you. George