Why won't some of my P3200 beds not go as low as others?

I have a number of Versacare P3200 beds but some won't go all the way down while others will?

crwdns2934089:0crwdne2934089:0 crwdns2934093:0crwdne2934093:0

crwdns2934109:0crwdne2934109:0

crwdns2889612:0crwdne2889612:0 0
crwdns2934285:0crwdne2934285:0