K.C.
I have had the same problem and think it is the detergent "sticking" to certain places on the clothes. OxiClean totally gets the stains out. Yes, it's a pain, but soak the item in OxiClean mixed with water... the box will give you ratios. And then rewash...
I now use a scoop of OxiClean in most of my loads and haven't had the "grease" problem in awhile.
Good luck!