My center needle comes unthreaded while sewing.

Question

Every time I try to use a stitch that uses two needles, the center needle comes unthreaded shortly after I begin stitching. I'm stumped...the thread comes out and the stitch uses the other three threads and continues smoothly. I can't figure out how it comes unthreaded without breaking! I've tried several needles and re-threaded the serger a million times, but no luck.