have you got at least 3 turns of string on your tuning posts, if not, the strings can slip back, especially the G,B and E strings.
Benny
July 2007
your tuner or tuners are bad, your neck is warped, due to inproper neck adjustment, they could be a number of problems. if this is not it, ask a local guitar dealer, most of the time they will give you a good clue, or ask you to bring your guitar in.