I am having problems with tracks in the mpc PLEASE HELP

Everytime I begin to record different instruments on each track in Seq. 1 in the mpc ( for instance track 1(drums) and next track 2(piano) etc... and so on) , The last track I recorded a instrument/melody on only plays back and when I go back to a previous track that I've already recorded a different instrument on, it doesn't play the track that's already been recorded on it , but it plays melody that I recorded on the last track or doesn't play at all? Shouldn't all of the tracks that I previously recorded with different melodies and instruments all play at the same time up until the last track I recorded when I press play start?

yup. make sure you didnt accedently switch one of your tracks off (f3 in the main screen) also play each track indivdually to see where you made a mistake.

or you might have SOLO pressed

Yes, it sounds like you might have SOLO on which will only play the track that is up on the screen. (Button F4)

Or you have muted tracks with button F3.

Scroll to the "On:" section of the track on MAIN SCREEN. Press OPEN WINDOW so you get the screen where you can switch tracks on & off with the pads. Track names with a black background are going to be off.

try togglin the pgms numbers for each track.