M/I usually means missing or invalid. Sometimes the reject is going to say M/I prior authorization meaning it needs the PA or you put in the wrong number. H315? What insurance does this reject usually pop out? Is it Medi-cal (if you're from CA)?
Usually if you cash it out and re-run the insurance again, it'll go through. =) (I'm a Waglreens Pharm Tech too.) The pharmacist actually taught me that trick. I didn't even think about it.
lol.. I didn't answer your question about what it means... All I know is how to fix the problem. Sorry