I've been vaccinated accordingly, but my mmr titer is coming back negative. what causes a person to not respond to the vaccine?

The way God made you: Some people respond well to a vaccine, serologically (which means something can be measured). Whereas others may not. MMR is actually quite a good vaccine, so I wonder if the vaccine was stored properly (needs to be kept cold), AND if they injected it right. You can always get a repeat shot and follow up titers (blood tests) to see if you respond serologically. An ID specialist may be able help u.