you need to give us some more details in order to advise you buddy , start with your stats, age, etc, what are you pinning and where , what pins are you using ?
This happened to me from lack of rotating and using delts too often(favorite spot). It could be scar tissue, or injecting too fast, not using the z-track method, or putting too many ml's in a smaller muscle...
Stats plz, at least age..in order to get any help...but.. make sure u take 10 secs per CC and then leave the needle in there for 10 more secs after ur done injecting and use an alcohol pad to gently massage the area when u take the needle out