Steve
If you are flying VNAV / LNAV engaged and you change vertical mode by pressing V/S or FLCH for descending, LNAV should remain engaged and the the aircraft should remain following the magenta.
If the aircraft stopped following the magenta while you engaged another vertical mode, something else happened unnoticed. If you see this behaviour again please capture and attach here a few screen captures of your forward cockpit to see what FMA a/p modes are telling, what the displays are telling, what the fmc is showing for diagnosing.