Repeated injections will cause the development of fibrosis and make subsequent injections more difficult. You may feel a bit of a "rip" or even hear the "rip" as you go through the tissue. Don't worry, just press on. If you can't get through, there may be calcification or excessive fibrosis secondary to trauma (from the needle). Natural process. No concerns. Stay with the upper outer quadrant, don't go "searching" for no scar tissue and hit your sciatic nerve, it may cause permenant paresthesias or weakness in the distal lower extremity. Grab a copy of Netter's Atlas of anatomy or do a search on yahoo for Gray's atlas (free online) and bone up on what you are doing, get a sense of which nerves supply which area, where to avoid, and the major vessels in the region.
NFG